Re: parity check for read?

On Tuesday April 3, mirko.benz@web.de wrote:

Hi,

Is parity calculation and validation for read operations supported?

I guess what you are asking is:

   With raid5, I would like the drive to handle a read request by
   reading all the blocks in the stripe and checking the parity.  If
   the parity is correct, return the data blocks.  If it is not, then
   fail the read request.  Is that possible?

No, it is not.

If you are really asking something else, please be more specific.

NeilBrown

Neil,

Exactly what I had in mind.

Some vendors claim they do parity checking for reads. Technically it should be possible for Linux RAID as well but is not implemented – correct?

Reliability data for unrecoverable read errors:
- enterprise SAS drive (ST3300655SS): 1 in 10^16 bits transfered, ~ 1 error in 1,1 PB
- enterprise SATA drive (ST3500630NS): 1 in 10^14 bits transfered, ~ 1 error in 11 TB

For a single SATA drive @ 50 MB/s it take on average 2,7 days to encounter an error.
For a large RAID with several drives this becomes much lower or am I viewing this wrong?

Regards,
Mirko

Neil Brown schrieb:

On Tuesday April 3, mirko.benz@web.de wrote:
  

quoted

Hi,

Is parity calculation and validation for read operations supported?
    

I guess what you are asking is:

   With raid5, I would like the drive to handle a read request by
   reading all the blocks in the stripe and checking the parity.  If
   the parity is correct, return the data blocks.  If it is not, then
   fail the read request.  Is that possible?

No, it is not.

If you are really asking something else, please be more specific.

NeilBrown

  

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Mirko Benz wrote:

Neil,

Exactly what I had in mind.

Some vendors claim they do parity checking for reads. Technically it should be possible for Linux RAID as well but is not implemented – correct?

Reliability data for unrecoverable read errors:
- enterprise SAS drive (ST3300655SS): 1 in 10^16 bits transfered, ~ 1 error in 1,1 PB
- enterprise SATA drive (ST3500630NS): 1 in 10^14 bits transfered, ~ 1 error in 11 TB

For a single SATA drive @ 50 MB/s it take on average 2,7 days to encounter an error.
For a large RAID with several drives this becomes much lower or am I viewing this wrong?

Regards,
Mirko

One note is that if the drive itself notices the unrecoverable read error, MD will see this as an IO error and rebuild the stripe.

What you need the parity check on read for is to validate errors not at the disk sector level, but rather ones that sneak in from DRAM, HBA errors or wire level uncorrected errors.

ric
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On 4/4/07, Ric Wheeler [off-list ref] wrote:

Mirko Benz wrote:

quoted

Neil,

Exactly what I had in mind.

Some vendors claim they do parity checking for reads. Technically it
should be possible for Linux RAID as well but is not implemented – correct?

Reliability data for unrecoverable read errors:
- enterprise SAS drive (ST3300655SS): 1 in 10^16 bits transfered, ~ 1
error in 1,1 PB
- enterprise SATA drive (ST3500630NS): 1 in 10^14 bits transfered, ~ 1
error in 11 TB

For a single SATA drive @ 50 MB/s it take on average 2,7 days to
encounter an error.
For a large RAID with several drives this becomes much lower or am I
viewing this wrong?

Regards,
Mirko

One note is that if the drive itself notices the unrecoverable read error, MD
will see this as an IO error and rebuild the stripe.

What you need the parity check on read for is to validate errors not at the disk
sector level, but rather ones that sneak in from DRAM, HBA errors or wire level
uncorrected errors.

For the raid5 case the corruption can be detected, but not corrected.
Is the expectation that the administrator will be notified to restore
from backup?

Are there any raid6 solutions that take advantage of what hpa has proved?
http://marc.info/?l=linux-raid&m=117333726129771&w=2

I am prototyping a writeback caching policy for MD, it seems plugging
in different read policies would be a straightforward extension.

ric

Dan
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