<div class="socmaildefaultfont" dir="ltr" style="font-family:Arial, Helvetica, sans-serif;font-size:9pt" ><div dir="ltr" >Olaf's explanation makes excellent sense.<br> </div>
<div dir="ltr" >So is this definitely the case? ..</div>
<div dir="ltr" > The now snapshotted inode is not updated when a live file is compressed, as it point to the current inode which itself has compressed blocks (and the compressed flag set). </div>
<div dir="ltr" > </div>
<div dir="ltr" >And likewise for deeper (older) snapshots.</div>
<div dir="ltr" ><div class="socmaildefaultfont" dir="ltr" style="font-family:Arial, Helvetica, sans-serif;font-size:10.5pt" ><div class="socmaildefaultfont" dir="ltr" style="font-family:Arial, Helvetica, sans-serif;font-size:10.5pt" ><div class="socmaildefaultfont" dir="ltr" style="font-family:Arial, Helvetica, sans-serif;font-size:10.5pt" ><div class="socmaildefaultfont" dir="ltr" style="font-family:Arial, Helvetica, sans-serif;font-size:10.5pt" ><div class="socmaildefaultfont" dir="ltr" ><div class="socmaildefaultfont" dir="ltr" ><div class="socmaildefaultfont" dir="ltr" ><div class="socmaildefaultfont" dir="ltr" ><div class="socmaildefaultfont" dir="ltr" ><div class="socmaildefaultfont" dir="ltr" ><div class="socmaildefaultfont" dir="ltr" ><div class="socmaildefaultfont" dir="ltr" ><div class="socmaildefaultfont" dir="ltr" ><div class="socmaildefaultfont" dir="ltr" ><div class="socmaildefaultfont" dir="ltr" ><div dir="ltr" style="margin-top: 20px;" ><div style="margin-top: 10px;" ><div style="font-family: sans-serif; font-size: 8pt; margin-bottom: 0.0001pt; line-height: normal;" ><span style="font-size:1.000em;" >sDaniel</span></div>
<div style="font-family: sans-serif; font-size: 8pt; margin-bottom: 0.0001pt; line-height: normal;" > </div>
<div style="margin-bottom: 0.0001pt; line-height: normal;" ><font color="#19A9F3" size="3" style="font-family: "Helvetica Neue", Helvetica, Arial, "Lucida Grande", sans-serif; font-size: 8pt; outline: none;" >_________________________________________________________</font><br style="outline: none;" ><font color="#808080" face="Arial" size="2" ><span style="caret-color: rgb(18, 18, 18);" ><b>Daniel Kidger</b></span></font></div>
<div style="margin-bottom: 0.0001pt; line-height: normal;" ><span style="outline: none; caret-color: rgb(18, 18, 18); color: rgb(18, 18, 18); font-size: 10.666666984558105px;" ><span style="outline: none; font-size: 7.5pt;" >IBM Technical Sales Specialist</span></span></div>
<div style="margin-bottom: 0.0001pt; line-height: normal;" ><span style="outline: none; caret-color: rgb(18, 18, 18); color: rgb(18, 18, 18); font-size: 10.666666984558105px;" ><span style="outline: none; font-size: 7.5pt;" >Spectrum Scale, Spectrum NAS and IBM Cloud Object Store<br style="outline: none;" ><br style="outline: none;" ><span style="font-size:0.857em;" >+</span></span></span><span style="font-size:0.857em;" ><span style="outline: none; font-family: sans-serif; color: rgb(95, 95, 95);" ><span style="outline: none; font-family: Verdana, Geneva, sans-serif;" ><span style="outline: none;" >44-(0)7818 522 266 </span></span></span></span><br style="outline: none; caret-color: rgb(18, 18, 18); color: rgb(18, 18, 18); font-family: sans-serif; font-size: 10.666666984558105px;" ><span style="outline: none; font-family: sans-serif; font-size: 10.666666984558105px; color: rgb(95, 95, 95);" ><span style="outline: none; font-family: Arial, Helvetica, sans-serif;" ><span style="outline: none; font-size: 8pt;" >daniel.kidger@uk.ibm.com</span></span></span>
<table border="0" style="border-collapse:collapse;border-color:#696969;table-layout:fixed;width:264px;word-wrap:break-word;" > <tbody> <tr> <td style="overflow: hidden; width: 88px; border-color : #696969;" ><a href="https://www.youracclaim.com/badges/687cf790-fe65-4a92-b129-d23ae41862ac/public_url" target="_blank"><img alt="" src="https://images.youracclaim.com/images/c49300ae-d13e-4071-90f5-15f59d199c9e/IBM%2BVolunteers%2BGold%2Bv6.png" style="width: 88px; height: 88px;" ></a></td> <td style="overflow: hidden; width: 88px; border-color : #696969;" ><a href="https://www.youracclaim.com/badges/8153c6a7-3e02-40be-87ee-24e27ae9459c/public_url" target="_blank"><img alt="" src="https://images.youracclaim.com/images/f2539224-f951-46b4-b376-b88f21c2be98/IBM-Selling-Certification---Level-1.png" style="width: 88px; height: 88px;" ></a></td> <td style="overflow: hidden; width: 88px; border-color : #696969;" ><a href="https://www.youracclaim.com/badges/78197e2c-4277-4ec9-808b-ad6abe1e1b16/public_url" target="_blank"><img alt="" src="https://images.youracclaim.com/images/ea52b12f-97ac-4e72-8d24-b0ced8054e7d/Storage%2BTechnical%2BV1.png" style="width: 88px; height: 88px;" ></a></td> </tr> </tbody></table>
<div> </div></div></div></div></div></div></div></div></div></div></div></div></div></div></div></div></div></div></div></div>
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<blockquote data-history-content-modified="1" dir="ltr" style="border-left:solid #aaaaaa 2px; margin-left:5px; padding-left:5px; direction:ltr; margin-right:0px" >----- Original message -----<br>From: "Olaf Weiser" <olaf.weiser@de.ibm.com><br>Sent by: gpfsug-discuss-bounces@spectrumscale.org<br>To: gpfsug main discussion list <gpfsug-discuss@spectrumscale.org><br>Cc:<br>Subject: [EXTERNAL] Re: [gpfsug-discuss] Compression question<br>Date: Thu, Nov 28, 2019 15:01<br> <br><br><br><font face="sans-serif" size="2" >Hi Alex, </font><br><font face="sans-serif" size="2" >not 100% sure about my answer.. but so far as I see it.. it is working, because of the so called "dito resolution " .. In the snaphost's inode .. die pointer to the DA's point the the next (more recent) inode information .. </font><br><font face="sans-serif" size="2" >so accessing a file in a snapshot- "redirects" the request to the origin inode - and there ..the information about compression is given and points to the origin DA</font><br><br><font face="sans-serif" size="2" >(of course.. only as long nobody changed the file since the snapshot was taken)</font><br><br><br><br><br><font color="#5f5f5f" face="sans-serif" size="1" >From: </font><font face="sans-serif" size="1" >"Alexander Wolf" <A.Wolf-Reber@de.ibm.com></font><br><font color="#5f5f5f" face="sans-serif" size="1" >To: </font><font face="sans-serif" size="1" >gpfsug-discuss@spectrumscale.org</font><br><font color="#5f5f5f" face="sans-serif" size="1" >Date: </font><font face="sans-serif" size="1" >11/28/2019 07:03 AM</font><br><font color="#5f5f5f" face="sans-serif" size="1" >Subject: </font><font face="sans-serif" size="1" >[EXTERNAL] Re: [gpfsug-discuss] Compression question</font><br><font color="#5f5f5f" face="sans-serif" size="1" >Sent by: </font><font face="sans-serif" size="1" >gpfsug-discuss-bounces@spectrumscale.org</font>
<hr noshade="noshade" ><br><br><br><font face="Arial" size="2" >I see the same behavior of mmlsattr on my system (with some post 5.0.4 development build). Funny enough if I look at the file content in the snapshot it gets properly decompressed.</font><br><font face="Arial" size="2" > </font><br><font face="Arial" size="2" >Mit freundlichen Grüßen / Kind regards</font>
<p><img src="cid:_4_DEB17BD4DEB177B80052596E872584C0" style="border:0px solid;" ></p>
<p> </p>
<table style="border-collapse:collapse;" width="697" > <tbody> <tr height="8" > <td style="border-style:none none none none;border-color:#000000;border-width:0px 0px 0px 0px;padding:0px 0px;" width="202" ><img alt="IBM Spectrum Scale" src="cid:_4_DEB18B14DEB187280052596E872584C0" style="border:0px solid;" ><br> <font face="sans-serif" size="2" > </font></td> <td style="border-style:none none none none;border-color:#000000;border-width:0px 0px 0px 0px;padding:0px 0px;" width="31" ><font face="sans-serif" size="3" > </font></td> <td style="border-style:none none none none;border-color:#000000;border-width:0px 0px 0px 0px;padding:0px 0px;" width="464" ><font face="Arial" size="3" ><b>Dr. Alexander Wolf-Reber</b></font><br> <font face="Arial" size="1" >Spectrum Scale Release Lead Architect<br> Department M069 / Spectrum Scale Software Development<br> <br> +49-160-90540880<br> a.wolf-reber@de.ibm.com</font></td> </tr> </tbody></table><br><img src="cid:_4_DEB19CD4DEB180B40052596E872584C0" style="border:0px solid;" ><br><a href="https://www.ibm.com/privacy/us/en/" target="_blank"><font color="blue" face="Arial" size="2" ><u>IBM Data Privacy Statement</u></font></a>
<p><font color="#a2a2a2" face="Arial" size="1" >IBM Deutschland Research & Development GmbH / Vorsitzende des Aufsichtsrats: Matthias Hartmann / Geschäftsführung: Dirk Wittkopp<br>Sitz der Gesellschaft: Böblingen / Registergericht: Amtsgericht Stuttgart, HRB 243294</font></p>
<p><font face="Arial" size="2" > </font><br><font face="Arial" size="2" > </font><br><font face="Arial" size="2" >----- Original message -----<br>From: "Luis Bolinches" <luis.bolinches@fi.ibm.com><br>Sent by: gpfsug-discuss-bounces@spectrumscale.org<br>To: "gpfsug main discussion list" <gpfsug-discuss@spectrumscale.org><br>Cc:<br>Subject: [EXTERNAL] Re: [gpfsug-discuss] Compression question<br>Date: Thu, Nov 28, 2019 14:00<br><br>Which version are you running? </font><br><font face="Arial" size="2" > </font><br><font face="Arial" size="2" >I was involved on a big for compressed file sets and snapshots that were related to what you see.<br> </font><br><font face="Arial" size="2" >-- </font><br><font face="Arial" size="2" >Cheers</font><br><font face="Arial" size="2" > </font><br><font face="Arial" size="2" >On 28. Nov 2019, at 14.57, Cregan, Bob <b.cregan@imperial.ac.uk> wrote:</font><br><br><br><font face="Calibri" size="3" >Hi </font><br><font face="Calibri" size="3" > Sounds logical - except the snap metadata does not have the compression flag set. So if the inode now points to a set of compressed blocks how does the client know to decompress it?</font><br><font face="Calibri" size="3" > </font><br><font face="Calibri" size="3" >After compression of an existing file we get in the snap</font><br><font face="Calibri" size="3" > </font><br><font face="Calibri" size="3" >-bash-4.2$ mmlsattr -L .snapshots/@GMT-2019.11.27-19.30.14/UserGuide_13.06.pdf</font><br><font face="Calibri" size="3" >file name: .snapshots/@GMT-2019.11.27-19.30.14/UserGuide_13.06.pdf</font><br><font face="Calibri" size="3" >metadata replication: 2 max 2</font><br><font face="Calibri" size="3" >data replication: 1 max 3</font><br><font face="Calibri" size="3" >immutable: no</font><br><font face="Calibri" size="3" >appendOnly: no</font><br><font face="Calibri" size="3" >flags: </font><br><font face="Calibri" size="3" >storage pool name: sata1</font><br><font face="Calibri" size="3" >fileset name: userdirs</font><br><font face="Calibri" size="3" >snapshot name: @GMT-2019.11.27-19.30.14</font><br><font face="Calibri" size="3" >creation time: Tue Mar 5 16:16:40 2019</font><br><font face="Calibri" size="3" >Misc attributes: ARCHIVE</font><br><font face="Calibri" size="3" >Encrypted: no</font></p>
<p><font face="Calibri" size="3" > </font><br><font face="Calibri" size="3" > and the original file is definitely compressed.</font><br><font face="Calibri" size="3" > </font><br><font face="Calibri" size="3" >-bash-4.2$ mmlsattr -L UserGuide_13.06.pdf </font><br><font face="Calibri" size="3" >file name: UserGuide_13.06.pdf</font><br><font face="Calibri" size="3" >metadata replication: 2 max 2</font><br><font face="Calibri" size="3" >data replication: 1 max 3</font><br><font face="Calibri" size="3" >immutable: no</font><br><font face="Calibri" size="3" >appendOnly: no</font><br><font face="Calibri" size="3" >flags: </font><br><font face="Calibri" size="3" >storage pool name: sata1</font><br><font face="Calibri" size="3" >fileset name: userdirs</font><br><font face="Calibri" size="3" >snapshot name: </font><br><font face="Calibri" size="3" >creation time: Tue Mar 5 16:16:40 2019</font><br><font face="Calibri" size="3" >Misc attributes: ARCHIVE COMPRESSION (library z)</font><br><font face="Calibri" size="3" >Encrypted: no</font></p>
<p><font face="Calibri" size="3" >Bob</font><br><font face="Calibri" size="3" > </font><br><font face="Calibri" size="3" > </font><br><font face="Calibri" size="3" > </font><br><font face="Calibri" size="3" > </font><br><font color="#0082bf" face="Arial" size="2" ><b>Bob Cregan</b></font><br><font face="Arial" size="2" >HPC Systems Analyst</font><br><font face="Arial" size="2" >Information & Communication Technologies</font><br><font face="Arial" size="2" >Imperial College London,<br>South Kensington Campus London, SW7 2AZ<br>T: 07712388129<br>E: b.cregan@imperial.ac.uk</font><br><font face="Arial" size="2" >W: </font><a href="http://www.imperial.ac.uk/ict/rcs" target="_blank"><font color="blue" face="Arial" size="2" ><u>www.imperial.ac.uk/ict/rcs</u></font></a><br><font color="#2f2f2f" face="Times New Roman" size="3" > </font></p>
<p><font face="Calibri" size="3" ><Outlook-1505984389.png></font><br><font face="Calibri" size="3" > </font><font face="Arial" size="2" >@imperialRCS @imperialRSE</font><font face="Calibri" size="3" > </font></p>
<p><font face="Calibri" size="3" > </font><br><font color="#2f2f2f" face="Times New Roman" size="3" > </font></p>
<p><font face="Calibri" size="3" ><Outlook-1505983882.png></font><br><font face="Calibri" size="3" > </font></p>
<p><font face="Calibri" size="3" > </font><br><font color="#2f2f2f" face="Times New Roman" size="3" > </font></p>
<p><font face="Calibri" size="3" > </font><br><font face="Calibri" size="3" > </font><br><font face="Calibri" size="3" > </font></p>
<p><font face="Arial" size="2" > </font></p>
<hr><br><font face="Calibri" size="2" ><b>From:</b> gpfsug-discuss-bounces@spectrumscale.org <gpfsug-discuss-bounces@spectrumscale.org> on behalf of Daniel Kidger <daniel.kidger@uk.ibm.com><br><b>Sent:</b> 28 November 2019 12:30<br><b>To:</b> gpfsug-discuss@spectrumscale.org <gpfsug-discuss@spectrumscale.org><br><b>Cc:</b> gpfsug-discuss@spectrumscale.org <gpfsug-discuss@spectrumscale.org><br><b>Subject:</b> Re: [gpfsug-discuss] Compression question</font><br><font face="Arial" size="2" > </font>
<table style="border-collapse:collapse;" width="630" > <tbody> <tr height="8" > <td bgcolor="#ffffcc" style="border-style:ridge ridge ridge ridge;border-color:#F0F0F0;border-width:3px 3px 3px 3px;padding:0px 0px;" width="622" ><font color="red" face="Arial" size="2" >Caution </font><font face="Arial" size="2" >- This email from daniel.kidger@uk.ibm.com originated outside Imperial </font></td> </tr> </tbody></table>
<p><font face="Arial" size="2" > </font></p>
<p><font face="Arial" size="1" >Alexander,</font><br><font face="Arial" size="1" > </font><br><font face="Arial" size="1" >Can you then confirm then that the inodes in the snapshot will now point to fewer but compressed blocks ?</font><br><font face="Arial" size="1" >Daniel</font><br><font face="Arial" size="1" > </font><br><font color="#00bfff" face="Arial" size="3" >_________________________________________________________</font><br><font color="#808080" face="Arial" size="2" ><b>Daniel Kidger</b></font><br><font face="Arial" size="1" >IBM Technical Sales Specialist</font><br><font face="Arial" size="1" >Spectrum Scale, Spectrum NAS and IBM Cloud Object Store</font><br><br><font face="Arial" size="1" >+</font><font color="#5f5f5f" face="Verdana" size="1" >44-(0)7818 522 266 </font><br><font color="#5f5f5f" face="Arial" size="1" >daniel.kidger@uk.ibm.com</font><font face="Arial" size="2" > </font></p>
<table style="border-collapse:collapse;" width="264" > <tbody> <tr height="8" > <td style="border-style:none none none none;border-color:#696969;border-width:0px 0px 0px 0px;padding:1px 1px;" width="86" > </td> <td style="border-style:none none none none;border-color:#696969;border-width:0px 0px 0px 0px;padding:1px 1px;" width="86" > </td> <td style="border-style:none none none none;border-color:#696969;border-width:0px 0px 0px 0px;padding:1px 1px;" width="86" > </td> </tr> </tbody></table><br><font face="Arial" size="2" > </font><br><font face="Arial" size="1" > </font><br><font face="Arial" size="1" > </font><br><font face="Arial" size="1" >----- Original message -----<br>From: "Alexander Wolf" <A.Wolf-Reber@de.ibm.com><br>Sent by: gpfsug-discuss-bounces@spectrumscale.org<br>To: gpfsug-discuss@spectrumscale.org<br>Cc:<br>Subject: [EXTERNAL] Re: [gpfsug-discuss] Compression question<br>Date: Thu, Nov 28, 2019 12:21<br> </font><br><font face="Arial" size="2" >I just tested this. Compressing a file did free up space in the file system. Looks like our compression code does not trigger COW on the snapshot. You can test this yourself by looking into mmlssnapshot -d (please not on a large production fs, this command is expensive).</font><br><font face="Arial" size="2" > </font><br><font face="Arial" size="2" >Mit freundlichen Grüßen / Kind regards</font>
<p><font face="Arial" size="2" > </font></p>
<p><font face="sans-serif" size="2" ><Image.15749424191280.png></font></p>
<p><font face="Arial" size="2" > </font></p>
<p> </p>
<table style="border-collapse:collapse;" width="724" > <tbody> <tr height="8" > <td style="border-style:none none none none;border-color:#000000;border-width:0px 0px 0px 0px;padding:0px 0px;" width="229" ><font face="sans-serif" size="2" ><Image.15749424191281.png></font><br> <font face="sans-serif" size="2" > </font></td> <td style="border-style:none none none none;border-color:#000000;border-width:0px 0px 0px 0px;padding:0px 0px;" width="31" ><font face="sans-serif" size="3" > </font></td> <td style="border-style:none none none none;border-color:#000000;border-width:0px 0px 0px 0px;padding:0px 0px;" width="464" ><font face="Arial" size="3" ><b>Dr. Alexander Wolf-Reber</b></font><br> <font face="Arial" size="1" >Spectrum Scale Release Lead Architect<br> Department M069 / Spectrum Scale Software Development<br> <br> +49-160-90540880<br> a.wolf-reber@de.ibm.com</font></td> </tr> </tbody></table><br><font face="sans-serif" size="2" ><Image.15749424191282.png></font><br><a href="https://www.ibm.com/privacy/us/en/" target="_blank"><font color="blue" face="Arial" size="2" ><u>IBM Data Privacy Statement</u></font></a>
<p><font color="#a2a2a2" face="Arial" size="1" >IBM Deutschland Research & Development GmbH / Vorsitzende des Aufsichtsrats: Matthias Hartmann / Geschäftsführung: Dirk Wittkopp<br>Sitz der Gesellschaft: Böblingen / Registergericht: Amtsgericht Stuttgart, HRB 243294</font></p>
<p><font face="Arial" size="2" > </font><br><font face="Arial" size="2" > </font><br><font face="Arial" size="2" >----- Original message -----<br>From: "Luis Bolinches" <luis.bolinches@fi.ibm.com><br>Sent by: gpfsug-discuss-bounces@spectrumscale.org<br>To: gpfsug-discuss@spectrumscale.org<br>Cc: gpfsug-discuss@spectrumscale.org<br>Subject: [EXTERNAL] Re: [gpfsug-discuss] Compression question<br>Date: Thu, Nov 28, 2019 11:47<br> </font><br><font face="Arial" size="2" >Hi</font><br><font face="Arial" size="2" > </font><br><font face="Arial" size="2" >Same principle COW. The data blocks do not get modified.</font><br><font color="#80803f" face="Arial" size="2" ><b>--<br>Ystävällisin terveisin / Kind regards / Saludos cordiales / Salutations / Salutacions<br>Luis Bolinches</b></font><br><font color="#80803f" face="Arial" size="2" ><b>Consultant IT Specialist<br>Mobile Phone: +358503112585</b></font><br><a href="https://www.youracclaim.com/user/luis-bolinches" target="_blank"><font color="blue" face="Arial" size="2" ><b><u>https://www.youracclaim.com/user/luis-bolinches</u></b></font></a><br><br><font color="#80803f" face="Arial" size="2" ><b>"If you always give you will always have" -- Anonymous</b></font><br><font face="Arial" size="1" > </font><br><font face="Arial" size="2" > </font><br><font face="Arial" size="2" > </font><br><font face="Arial" size="2" >----- Original message -----<br>From: "Cregan, Bob" <b.cregan@imperial.ac.uk><br>Sent by: gpfsug-discuss-bounces@spectrumscale.org<br>To: gpfsug main discussion list <gpfsug-discuss@spectrumscale.org><br>Cc:<br>Subject: [EXTERNAL] Re: [gpfsug-discuss] Compression question<br>Date: Thu, Nov 28, 2019 12:06 PM<br> </font><br><font face="Calibri" size="3" >Just to clarify this is SS compression, so </font><br><font face="Calibri" size="3" > </font><br><font face="Calibri" size="3" >mmchattr --compression yes <filename></font><br><font face="Calibri" size="3" > </font><br><font face="Calibri" size="3" >or an ILM equivalent</font><br><font face="Calibri" size="3" > </font><br><font face="Calibri" size="3" >So not a regular modification.</font><br><font face="Calibri" size="3" > </font><br><font face="Calibri" size="3" >Bob</font><br><font face="Calibri" size="3" > </font><br><font face="Calibri" size="3" > </font><br><font face="Calibri" size="3" > </font><br><font color="#0082bf" face="Arial" size="2" ><b>Bob Cregan</b></font><br><font face="Arial" size="2" >HPC Systems Analyst</font><br><font face="Arial" size="2" >Information & Communication Technologies</font><br><font face="Arial" size="2" >Imperial College London,<br>South Kensington Campus London, SW7 2AZ<br>T: 07712388129<br>E: b.cregan@imperial.ac.uk</font><br><font face="Arial" size="2" >W: </font><a href="http://www.imperial.ac.uk/ict/rcs" target="_blank"><font color="blue" face="Arial" size="2" ><u>www.imperial.ac.uk/ict/rcs</u></font></a><br><font color="#2f2f2f" face="Times New Roman" size="3" > </font></p>
<p><font face="Calibri" size="3" ><Image.f7e9a6a7-c4ab-4a2c-a979-4513774aa054.png></font><br><font face="Calibri" size="3" > </font><font face="Arial" size="2" >@imperialRCS @imperialRSE</font><font face="Calibri" size="3" > </font></p>
<p><font face="Calibri" size="3" > </font><br><font color="#2f2f2f" face="Times New Roman" size="3" > </font></p>
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<hr><br><font face="Calibri" size="2" ><b>From:</b> Cregan, Bob<br><b>Sent:</b> 28 November 2019 09:43<br><b>To:</b> gpfsug main discussion list <gpfsug-discuss@spectrumscale.org><br><b>Subject:</b> Compression question</font><font face="Arial" size="2" > </font><br><font face="Arial" size="2" > </font><br><font face="Calibri" size="3" >Hi All,</font><br><font face="Calibri" size="3" > Can someone answer the following question on compression in a snapshot context? We have tried various experiments and they are inconclusive - too tedious to go into the detail.</font><br><font face="Calibri" size="3" > </font><br><font face="Calibri" size="3" >What happens to the snapshot when a file is compressed in SS? The logic as I see it is </font><br><font face="Calibri" size="3" > </font><br><font face="Calibri" size="3" >####### In a non compressed situation ###############</font><br><font face="Calibri" size="3" > </font><br><font face="Calibri" size="3" >1) create a file,</font><br><font face="Calibri" size="3" >2) create a snapshot.</font><br><font face="Calibri" size="3" >3) modify a file in a normal way - the blocks on disk are changed and the old blocks are then written to the snap. </font><br><font face="Calibri" size="3" > </font><br><font face="Calibri" size="3" >######In a compressed situation ############</font><br><font face="Calibri" size="3" > </font><br><font face="Calibri" size="3" >1) create a file,</font><br><font face="Calibri" size="3" >2) create a snapshot.</font><br><font face="Calibri" size="3" >3) Compress the file. Now IBM says the blocks are rewritten when the file is compressed. So do the old uncompressed blocks go into the snap? If so we now have 2 copies of the file and unless the compression > 50% we have used more space until the snap is deleted.</font><br><font face="Calibri" size="3" > </font><br><font face="Calibri" size="3" >You get the space back in the end, but if you are in a tight situation then potentially compression might not work for you in the short term.</font><br><font face="Calibri" size="3" >Thanks</font><br><font face="Calibri" size="3" > </font><br><font face="Calibri" size="3" >Bob</font><br><font face="Calibri" size="3" > </font><br><font face="Calibri" size="3" > </font><br><font color="#0082bf" face="Arial" size="2" ><b>Bob Cregan</b></font><br><font face="Arial" size="2" >HPC Systems Analyst</font><br><font face="Arial" size="2" >Information & Communication Technologies</font><br><font face="Arial" size="2" >Imperial College London,<br>South Kensington Campus London, SW7 2AZ<br>T: 07712388129<br>E: b.cregan@imperial.ac.uk</font><br><font face="Arial" size="2" >W: </font><a href="http://www.imperial.ac.uk/ict/rcs" target="_blank"><font color="blue" face="Arial" size="2" ><u>www.imperial.ac.uk/ict/rcs</u></font></a><br><font color="#2f2f2f" face="Times New Roman" size="3" > </font>
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