Neil Brown wrote:
On Tuesday March 6, rabbit@xxxxxxxxx wrote:
Neil Brown wrote:
When we write to a raid1, the data is DMAed from memory out to each
device independently, so if the memory changes between the two (or
more) DMA operations, you will get inconsistency between the devices.
Does this apply to raid 10 devices too? And in case of LVM if swap is on
top of a LV which is a part of a VG which has a single PV as the raid
array - will this happen as well? Or will the LVM layer take the data
once and distribute exact copies of it to the PVs (in this case just the
raid) effectively giving the raid array invariable data?
Yes, it applies to raid10 too.
I don't know the details of the inner workings of LVM, but I doubt it
will make a difference. Copying the data in memory is just too costly
to do if it can be avoided. With LVM and raid1/10 it can be avoided
with no significant cost.
With raid4/5/6, not copying into the cache can cause data corruption.
So we always copy.
I see. So basically for those of us who want to run swap on raid 1 or
10, and at the same time want to rely on mismatch_cnt for early problem
detection, the only option is to create a separate md device just for
the swap. Is this about right?
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