[Gluster-users] Quick question regarding xfs_repair

[Mohit Anchlia]

Thanks! Can you point me to best practices or place where I can read
more about it and decide on best practices. I am assuming there are
other things to worry about :) Knowing about those will help in
atleast having some process ready or in testing it out. We currently
use NFS (single point of failure) but will be moving to local disks
which isn't maintained by storage team so far.

On Mon, Mar 14, 2011 at 10:08 AM, Joe Landman
<landman at scalableinformatics.com> wrote:
> On 03/14/2011 01:02 PM, Mohit Anchlia wrote:
>>
>> What RAID level is this? Would RAID 10 or RAID 6 help in such scenarios?
>
> Not so much ... if the underlying block device gets corruption, the file
> system atop it might not be able to survive writing to or reading from a
> corrupt region.  RAID6 could work if you have a read after write function
> built into the RAID to guarantee that what gets written is what was
> committed to disk.  Most (actually all) hardware on the market at the moment
> doesn't have this.  Some do talk about capability like this, but the real
> test is to take the write, cache it, compute md5 or similar sums, perform
> the write, then force a non-cached re-read of the data, and compute then
> compare the md5 or similar sums.  If the sums are incorrect, then
> reallocating the write to another location and iterating this.
>
> Technically, GlusterFS could do this in a translator.  Realistically, this
> would kill performance.
>
> RAID10 doesn't compute parity, so you need checksums to see if the data is
> correct.  RAID6 does compute parity (2 different mechanisms), but the
> implementations can't really identify a failed write/read, as the parity is
> recomputed at write time, so if corruption occurred in the RAID card (say a
> failing cache chip, or data cable, or ...), its possible bad data was
> committed, and the parity data wouldn't reflect a problem.
>
> Regards,
>
> Joe
>
>
> --
> Joseph Landman, Ph.D
> Founder and CEO
> Scalable Informatics Inc.
> email: landman at scalableinformatics.com
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