Question on RAID-5 rmw & rcw(read modify write & read construct write)

Tirumala Reddy Marri

2009-07-20 18:00:05 UTC

Hi Neil,
I am reading following lines from "raid5.h" , not clear what they
mean. If some DMA engines doesn't support what should we do ?

* 1/ Parity check operations destroy the in cache version of the parity
block,
* so we prevent parity dependent operations like writes and
compute_blocks
* from starting while a check is in progress. Some dma engines can
perform
* the check without damaging the parity block, in these cases the
parity
* block is re-marked up to date (assuming the check was successful)
and is
* not re-read from disk.

Regards,
Marri

-----Original Message-----
From: linux-raid-***@vger.kernel.org
[mailto:linux-raid-***@vger.kernel.org] On Behalf Of Tirumala Reddy
Marri
Sent: Monday, July 20, 2009 10:39 AM
To: linux-***@vger.kernel.org
Subject: Question on RAID-5 rmw & rcw(read modify write & read construct
write)

Hi,
I am trying to understand read-modify-write & read-construct-write as
part of my HW accelerated RAID-5 driver debugging. How it is decided on
stripe to be rmw/rcw ?
Is this happen every time when application try to write to disks with
bio of less than a chunk size ?

I am seeing file system inconsistencies while raid-5 recovery is
happening and writing data to /dev/md0 with ext3/ext2 installed and
mounted on a directory.

Thanks in advance,
Marri
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