[CentOS] question about software Raid 1

Sun Sep 21 18:46:47 UTC 2008
Nataraj <incoming-centos at rjl.com>

On Sun, 2008-09-21 at 11:12 -0700, John R Pierce wrote:
> Nataraj wrote:
> > Does software raid 1 compare checksums or otherwise verify that the same
> > bits are coming from both disks during reads?  What I'm interested in,
> > is whether bit errors that were somehow undetected by the hardware would
> > be detected by the raid 1 software.
> >   
> under normal operation, each read request goes to one or the other 
> drive, this doubles the read throughput as both drives can be servicing 
> different read requests at the same time.
> some raid does a scrub, where in the background, when the disks are 
> otherwise idle, it gradually reads all the raid stripes and validates 
> them.    I honestly don't know if Linux built in raid does this or 
> not.   Of course, with RAID-1, if the two blocks disagree, there's no 
> way of knowing which one is correct, only that there is a potential problem.
> Some raid (Sun ZFS, for instance) stores a checksum with every block so 
> it can detect corruption immediately.    Also, I know ZFS does this 
> scrubbing.

Thank you John.  I'm pretty sure that raid 5 or 6 would be safe, since
there is parity checking, however it sounds like this may not be the
case for raid 1.

Over the years, one of the things i've noticed is that whenever I've
seen a SCSI drive fail, there have always been hardware exceptions from
the drive.  With less expensive ATA drives (I can't say weather this
includes current generation SATA drives), I have seen quite a few drives
fail in such a way that they simply returned unreliable data (you could
read the same data twice and get 2 different results) without raising
any hardware exceptions.

So for this reason, I have been reluctant to use inexpensive SATA
drives, unless there is an easy way to know that the drive is returning
accurate data.

As my data has grown, I've been more challenged to come up with
affordable backup solutions that I feel confident in.  Recently I
started backing up to a pair of usb/esata terabyte drives with 2 .5
terrabyte partitions on each drive, running software raid 1 across the
two partitions.

Note that I am mostly talking about backups here.  My primary data is
mostly on RAID5 or RAID10 U350 SCSI arrays.