<div class="socmaildefaultfont" dir="ltr" style="font-family:Arial, Helvetica, sans-serif;font-size:10pt" ><div dir="ltr" style="font-family:Arial, Helvetica, sans-serif;font-size:10pt" ><div dir="ltr" >Hi</div>
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<div dir="ltr" >snaps are copy on write (COW). No blocks are written to snap compressed or not compressed situation</div>
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<div dir="ltr" >###</div>
<div dir="ltr" >Create a file</div>
<div dir="ltr" >Take snap</div>
<div dir="ltr" >Modify file, only the blocks that are modified are linked from the active i-nodes, snap i-nodes point to the original blocks. you only free space when a data block has no snaps (and active) pointers from i-nodes</div>
<div dir="ltr" >###</div>
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<div dir="ltr" >That is the same if data blocks are compressed or not</div>
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<div dir="ltr" >Hope that helps</div>
<div dir="ltr" ><div dir="ltr" style="font-family:Arial, Helvetica, sans-serif;font-size:10pt" ><div dir="ltr" style="font-family:Arial, Helvetica, sans-serif;font-size:10pt" ><div dir="ltr" style="font-family:Arial, Helvetica, sans-serif;font-size:10.5pt" ><div dir="ltr" style="font-family:Arial, Helvetica, sans-serif;font-size:10.5pt" ><div dir="ltr" style="font-family:Arial, Helvetica, sans-serif;font-size:10.5pt" ><div dir="ltr" ><div style="font-size: 12pt; font-weight: bold; font-family: sans-serif; color: #7C7C5F;" ><span style="font-size:10pt;" ><font face="Arial" >--<br>Ystävällisin terveisin / Kind regards / Saludos cordiales / Salutations</font> / Salutacions<br><font face="Arial" >Luis Bolinches</font></span></div>
<div style="font-size: 12pt; font-weight: bold; font-family: sans-serif; color: #7C7C5F;" ><span style="font-size:10pt;" ><font face="Arial" >Consultant IT Specialist</font><br><font face="Arial" >Mobile Phone: +358503112585</font><br><a href="https://www.youracclaim.com/user/luis-bolinches" rel="noopener" target="_blank"><b><font face="Arial" >https://www.youracclaim.com/user/luis-bolinches</font></b></a><br><br><b><font face="Arial" color="#7C7C5F" >"If you always give you will always have" -- Anonymous</font></b></span></div>
<div style="font-size: 8pt; font-family: sans-serif; margin-top: 10px;" ><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: "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] [gpfsug-discuss] Compression question<br>Date: Thu, Nov 28, 2019 11:44 AM<br> <br><!--Notes ACF
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<div style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);" >Hi All,</div>
<div style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);" > 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.</div>
<div style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);" > </div>
<div style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);" ><div style="margin: 0px; font-size: 12pt; font-family: Calibri, Arial, Helvetica, sans-serif; color: black; background-color: rgb(255, 255, 255)" >What happens to the snapshot when a file is compressed in SS? The logic as I see it is </div>
<div style="margin: 0px; font-size: 12pt; font-family: Calibri, Arial, Helvetica, sans-serif; color: black; background-color: rgb(255, 255, 255)" > </div>
<div style="margin: 0px; font-size: 12pt; font-family: Calibri, Arial, Helvetica, sans-serif; color: black; background-color: rgb(255, 255, 255)" >####### In a non compressed situation ###############</div>
<div style="margin: 0px; font-size: 12pt; font-family: Calibri, Arial, Helvetica, sans-serif; color: black; background-color: rgb(255, 255, 255)" > </div>
<div style="margin: 0px; font-size: 12pt; font-family: Calibri, Arial, Helvetica, sans-serif; color: black; background-color: rgb(255, 255, 255)" >1) create a file,</div>
<div style="margin: 0px; font-size: 12pt; font-family: Calibri, Arial, Helvetica, sans-serif; color: black; background-color: rgb(255, 255, 255)" >2) create a snapshot.</div>
<div style="margin: 0px; font-size: 12pt; font-family: Calibri, Arial, Helvetica, sans-serif; color: black; background-color: rgb(255, 255, 255)" >3) modify a file in a normal way - the blocks on disk are changed and the old blocks are then written to the snap. </div>
<div style="margin: 0px; font-size: 12pt; font-family: Calibri, Arial, Helvetica, sans-serif; color: black; background-color: rgb(255, 255, 255)" > </div>
<div style="margin: 0px; font-size: 15px; font-family: "Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, system-ui, Roboto, "Helvetica Neue", sans-serif; color: rgb(32, 31, 30); background-color: rgb(255, 255, 255)" ><div dir="ltr" style="margin: 0px; font-size: 12pt; font-family: Calibri, Helvetica, sans-serif, Helvetica, EmojiFont, "Apple Color Emoji", "Segoe UI Emoji", NotoColorEmoji, "Segoe UI Symbol", "Android Emoji", EmojiSymbols; color: black" ><p style="margin-top: 0px; margin-bottom: 0px" >######In a compressed situation ############</p>
<p style="margin-top: 0px; margin-bottom: 0px" > </p>
<div style="margin: 0px" ><div style="margin: 0px; font-family: Calibri, Arial, Helvetica, sans-serif" >1) create a file,</div>
<div style="margin: 0px; font-family: Calibri, Arial, Helvetica, sans-serif" >2) create a snapshot.</div>
<div style="margin: 0px; font-family: Calibri, Arial, Helvetica, sans-serif" >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.</div>
<div style="margin: 0px; font-family: Calibri, Arial, Helvetica, sans-serif" > </div>
<div style="margin: 0px; font-family: Calibri, Arial, Helvetica, sans-serif" >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.</div></div></div></div></div>
<div style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);" >Thanks</div>
<div style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);" > </div>
<div style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);" >Bob</div>
<div id="Signature" ><div dir="ltr" id="divtagdefaultwrapper" style="font-size:12pt; color:rgb(0,0,0); font-family:Calibri,Helvetica,sans-serif,Helvetica,EmojiFont,"Apple Color Emoji","Segoe UI Emoji",NotoColorEmoji,"Segoe UI Symbol","Android Emoji",EmojiSymbols" ><p style="margin-top: 0px; margin-bottom: 0px;" > </p>
<p style="margin-top: 0px; margin-bottom: 0px;" > </p>
<p style="margin-top: 0px; margin-bottom: 0px;margin:0cm 0cm 0.0001pt; font-size:12pt; font-family:"Times New Roman",serif; color:rgb(33,33,33)" ><strong><span style="font-family:Arial,sans-serif,serif,EmojiFont; color:rgb(0,111,201); font-size:10pt" >Bob Cregan</span></strong><br><span style="font-family:Arial,sans-serif,serif,EmojiFont; color:black; font-size:10pt" >HPC Systems Analyst</span></p>
<p style="margin-top: 0px; margin-bottom: 0px;margin:0cm 0cm 0.0001pt; font-size:12pt; font-family:"Times New Roman",serif; color:rgb(33,33,33)" ><span style="font-family:Arial,sans-serif,serif,EmojiFont; color:black; font-size:10pt" >Information & Communication Technologies</span></p>
<p style="margin-top: 0px; margin-bottom: 0px;margin:0cm 0cm 0.0001pt; font-size:12pt; font-family:"Times New Roman",serif; color:rgb(33,33,33)" ><span style="font-family:Arial,sans-serif,serif,EmojiFont; color:black" ><span style="font-size:10pt" >Imperial College London, </span><br><span style="font-size:10pt" >South Kensington Campus London, SW7 2AZ</span><br><span style="font-size:10pt" >T: 07712388129</span><br><span style="font-size:10pt" >E: b.cregan@imperial.ac.uk</span></span></p>
<p style="margin-top: 0px; margin-bottom: 0px;margin:0cm 0cm 0.0001pt; font-size:12pt; font-family:"Times New Roman",serif; color:rgb(33,33,33)" ><span style="font-family:Arial,sans-serif,serif,EmojiFont; color:black" ><span style="font-size:10pt" >W: </span><a href="http://www.imperial.ac.uk/ict/rcs" id="LPNoLP" rel="noopener noreferrer" target="_blank"><span style="font-size:10pt" >www.imperial.ac.uk/</span><span style="font-size:10pt" >ict</span><span style="font-size:10pt" >/</span><span style="font-size:10pt" >rcs</span></a></span></p>
<p style="margin-top: 0px; margin-bottom: 0px;margin:0cm 0cm 0.0001pt; font-size:12pt; font-family:"Times New Roman",serif; color:rgb(33,33,33); line-height:24px" ><sub><span style="font-size:10pt; line-height:23px; font-family:Calibri,sans-serif,serif,EmojiFont; color:rgb(31,73,125)" ><img alt="1505984389175_twitter.png" data-outlook-trace="F:1|T:1" src="cid:ba38b5d6-12d6-476a-bcd8-7d647bfab5b3" style="" > </span></sub><span style="font-size:10pt; line-height:23px; font-family:Arial,sans-serif,serif,EmojiFont; color:black" >@imperialRCS @imperialRSE</span></p>
<p style="margin-top: 0px; margin-bottom: 0px;margin:0cm 0cm 0.0001pt; font-size:12pt; font-family:"Times New Roman",serif; color:rgb(33,33,33)" ><img alt="1505983882959_Imperial-RCS.png" data-outlook-trace="F:1|T:1" src="cid:e34493c2-d35b-4fff-a6df-ce87e2ae5ed4" style="" ></p>
<p style="margin-top: 0px; margin-bottom: 0px;margin:0cm 0cm 0.0001pt; font-size:12pt; font-family:"Times New Roman",serif; color:rgb(33,33,33); line-height:24px" > </p>
<p style="margin-top: 0px; margin-bottom: 0px;" > </p>
<p style="margin-top: 0px; margin-bottom: 0px;" > </p></div></div>
<div><font size="2" face="Default Monospace,Courier New,Courier,monospace" >_______________________________________________<br>gpfsug-discuss mailing list<br>gpfsug-discuss at spectrumscale.org<br><a href="http://gpfsug.org/mailman/listinfo/gpfsug-discuss" target="_blank">http://gpfsug.org/mailman/listinfo/gpfsug-discuss</a> </font></div></blockquote>
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