[dm-devel] Desynchronizing dm-raid1
Martin K. Petersen
martin.petersen at oracle.com
Thu Apr 3 14:49:19 UTC 2008
>>>>> "Malahal" == malahal <malahal at us.ibm.com> writes:
>> 1. lock the buffers and unmap the pages while they are being
>> written. --- upstream developers would likely reject it. No other
>> driver than dm-raid1 has problems with this and they wouldn't damp
>> performance because of one driver.
Malahal> Very few drivers require it, so how about an interface to
Malahal> lock the pages of an I/O available to drivers. Only needed
Malahal> RAID drivers would lock the I/O while it is in progress and
Malahal> they only pay the performance penalty. mmap pages are a bit
Malahal> tricky. They need to go into read-only mode when an I/O is in
Malahal> progress. I know this would likely be rejected too!!!
I have exactly the same problem with the data integrity stuff I'm
working on.
Essentially a checksum gets generated when a bio is submitted and both
the I/O controller and the disk drive verify the checksum.
With ext2 in particular I often experience that the page (usually
containing directory metadata) has been modified before the controller
does the DMA. And the I/O will then be rejected by the controller or
drive because the checksum doesn't match the data.
So this problem is not specific to DM/MD...
--
Martin K. Petersen Oracle Linux Engineering
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