[gpfsug-discuss] FW: [EXTERNAL] FLASH: IBM Spectrum Scale (GPFS) V4.1 and 4.2 levels: network reconnect function may result in file system corruption or undetected file data corruption (2017.10.09)

Tue Oct 10 16:09:20 BST 2017

Bob,

The problem may occur when the TCP connection is broken between two nodes. 
While in the vast majority of the cases when data stops flowing through 
the connection, the result is one of the nodes getting expelled, there are 
cases where the TCP connection simply breaks -- that is relatively rare 
but happens on occasion. There is logic in the mmfsd daemon to detect the 
disconnection and attempt to reconnect to the destination in question. If 
the reconnect is successful then steps are taken to recover the state kept 
by the daemons, and that includes resending some RPCs that were in flight 
when the disconnection took place.

As the flash describes, a problem in the logic to resend some RPCs was 
causing one of the RPC headers to be omitted, resulting in the RPC data to 
be interpreted as the (missing) header. Normally the result is an assert 
on the receiving end, like the "logAssertFailed: !"Request and queue size 
mismatch"  assert described in the flash. However, it's at least 
conceivable (though expected to very rare) that the content of the RPC 
data could be interpreted as a valid RPC header. In the case of an RPC 
which involves data transfer between an NSD client and NSD server, that 
might result in incorrect data being written to some NSD device.

Disconnect/reconnect scenarios appear to be uncommon. An entry like

[N] Reconnected to xxx.xxx.xxx.xxx nodename <c0n0> 

in mmfs.log would be an indication that a reconnect has occurred. By 
itself, the reconnect will not imply that data or the file system was 
corrupted, since that will depend on what RPCs were pending when the 
connection happened. In the case the assert above is hit, no corruption is 
expected, since the daemon will go down before incorrect data gets 
written.

Reconnects involving an NSD server are those which present the highest 
risk, given that NSD-related RPCs are used to write data into NSDs

Even on clusters that have not been subjected to disconnects/reconnects 
before, such events might still happen in the future in case of network 
glitches. It's then recommended that an efix for the problem be applied in 
a timely fashion.

Reference: http://www-01.ibm.com/support/docview.wss?uid=ssg1S1010668

Regards, The Spectrum Scale (GPFS) team

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From:   "Oesterlin, Robert" <Robert.Oesterlin at nuance.com>
To:     gpfsug main discussion list <gpfsug-discuss at spectrumscale.org>
Date:   10/09/2017 10:38 AM
Subject:        [gpfsug-discuss] FW: [EXTERNAL] FLASH: IBM Spectrum Scale 
(GPFS) V4.1 and 4.2 levels: network reconnect function may result in file 
system corruption or undetected file data corruption (2017.10.09)
Sent by:        gpfsug-discuss-bounces at spectrumscale.org

Can anyone from the Scale team comment? 

Anytime I see “may result in file system corruption or undetected file 
data corruption” it gets my attention.

Bob Oesterlin
Sr Principal Storage Engineer, Nuance

Storage 
IBM My Notifications 
Check out the IBM Electronic Support 

IBM Spectrum Scale 

: IBM Spectrum Scale (GPFS) V4.1 and 4.2 levels: network reconnect 
function may result in file system corruption or undetected file data 
corruption 

IBM has identified a problem with IBM Spectrum Scale (GPFS) V4.1 and V4.2 
levels, in which resending an NSD RPC after a network reconnect function 
may result in file system corruption or undetected file data corruption. 

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