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    SubjectRe: Wrong DIF guard tag on ext2 write
    On Fri, Jul 23, 2010 at 3:16 PM, Vladislav Bolkhovitin <> wrote:
    > Gennadiy Nerubayev, on 07/23/2010 09:59 PM wrote:
    >> On Thu, Jun 3, 2010 at 7:20 AM, Vladislav Bolkhovitin<>
    >>  wrote:
    >>> James Bottomley, on 06/01/2010 05:27 PM wrote:
    >>>> On Tue, 2010-06-01 at 12:30 +0200, Christof Schmitt wrote:
    >>>>> What is the best strategy to continue with the invalid guard tags on
    >>>>> write requests? Should this be fixed in the filesystems?
    >>>> For write requests, as long as the page dirty bit is still set, it's
    >>>> safe to drop the request, since it's already going to be repeated.  What
    >>>> we probably want is an error code we can return that the layer that sees
    >>>> both the request and the page flags can make the call.
    >>>>> Another idea would be to pass invalid guard tags on write requests
    >>>>> down to the hardware, expect an "invalid guard tag" error and report
    >>>>> it to the block layer where a new checksum is generated and the
    >>>>> request is issued again. Basically implement a retry through the whole
    >>>>> I/O stack. But this also sounds complicated.
    >>>> No, no ... as long as the guard tag is wrong because the fs changed the
    >>>> page, the write request for the updated page will already be queued or
    >>>> in-flight, so there's no need to retry.
    >>> There's one interesting problem here, at least theoretically, with SCSI
    >>> or similar transports which allow to have commands queue depth>1 and allowed
    >>> to internally reorder queued requests. I don't know the FS/block layers
    >>> sufficiently well to tell if sending several requests for the same page
    >>> really possible or not, but we can see a real life problem, which can be
    >>> well explained if it's possible.
    >>> The problem could be if the second (rewrite) request (SCSI command) for
    >>> the same page queued to the corresponding device before the original request
    >>> finished. Since the device allowed to freely reorder requests, there's a
    >>> probability that the original write request would hit the permanent storage
    >>> *AFTER* the retry request, hence the data changes it's carrying would be
    >>> lost, hence welcome data corruption.
    >>> For single parallel SCSI or SAS devices such race may look practically
    >>> impossible, but for sophisticated clusters when many nodes pretending to be
    >>> a single SCSI device in a load balancing configuration, it becomes very
    >>> real.
    >>> The real life problem we can see in an active-active DRBD-setup. In this
    >>> configuration 2 nodes act as a single SCST-powered SCSI device and they both
    >>> run DRBD to keep their backstorage in-sync. The initiator uses them as a
    >>> single multipath device in an active-active round-robin load-balancing
    >>> configuration, i.e. sends requests to both nodes in parallel, then DRBD
    >>> takes care to replicate the requests to the other node.
    >>> The problem is that sometimes DRBD complies about concurrent local
    >>> writes, like:
    >>> kernel: drbd0: scsi_tgt0[12503] Concurrent local write detected! [DISCARD
    >>> L] new: 144072784s +8192; pending: 144072784s +8192
    >>> This message means that DRBD detected that both nodes received
    >>> overlapping writes on the same block(s) and DRBD can't figure out which one
    >>> to store. This is possible only if the initiator sent the second write
    >>> request before the first one completed.
    >>> The topic of the discussion could well explain the cause of that. But,
    >>> unfortunately, people who reported it forgot to note which OS they run on
    >>> the initiator, i.e. I can't say for sure it's Linux.
    >> Sorry for the late chime in, but here's some more information of
    >> potential interest as I've previously inquired about this to the drbd
    >> mailing list:
    >> 1. It only happens when using blockio mode in IET or SCST. Fileio,
    >> nv_cache, and write_through do not generate the warnings.
    > Some explanations for those who not familiar with the terminology:
    >  - "Fileio" means Linux IO stack on the target receives IO via
    > vfs_readv()/vfs_writev()
    >  - "NV_CACHE" means all the cache synchronization requests
    > (SYNCHRONIZE_CACHE, FUA) from the initiator are ignored
    >  - "WRITE_THROUGH" means write through, i.e. the corresponding backend file
    > for the device open with O_SYNC flag.
    >> 2. It happens on active/passive drbd clusters (on the active node
    >> obviously), NOT active/active. In fact, I've found that doing round
    >> robin on active/active is a Bad Idea (tm) even with a clustered
    >> filesystem, until at least the target software is able to synchronize
    >> the command state of either node.
    >> 3. Linux and ESX initiators can generate the warning, but I've so far
    >> only been able to reliably reproduce it using a Windows initiator and
    >> sqlio or iometer benchmarks. I'll be trying again using iometer when I
    >> have the time.
    >> 4. It only happens using a random write io workload (any block size),
    >> with initiator threads>1, OR initiator queue depth>1. The higher
    >> either of those is, the more spammy the warnings become.
    >> 5. The transport does not matter (reproduced with iSCSI and SRP)
    >> 6. If DRBD is disconnected (primary/unknown), the warnings are not
    >> generated. As soon as it's reconnected (primary/secondary), the
    >> warnings will reappear.
    > It would be great if you prove or disprove our suspicions that Linux can
    > produce several write requests for the same blocks simultaneously. To be
    > sure we need:
    > 1. The initiator is Linux. Windows and ESX are not needed for this
    > particular case.
    > 2. If you are able to reproduce it, we will need full description of which
    > application used on the initiator to generate the load and in which mode.
    > Target and DRBD configuration doesn't matter, you can use any.

    I just tried, and this particular DRBD warning is not reproducible
    with io (iometer) coming from a Linux initiator ( The same
    iometer parameters were used as on windows, and both the base device
    as well as filesystem (ext3) were tested, both negative. I'll try a
    few more tests, but it seems that this is a nonissue with a Linux

    Hope that helps,

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      Last update: 2010-07-23 22:55    [W:0.031 / U:6.440 seconds]
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