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md.4: various improvements to new section on scrubbing. 

Signed-off-by: NeilBrown <neilb@suse.de>
This commit is contained in:
NeilBrown 2010-01-29 10:21:56 +11:00
parent 417a4b046d
commit c93e9d68d0
1 changed files with 18 additions and 21 deletions
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md.4
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@ -430,14 +430,12 @@ in the
.I sysfs
directory for the device.
Writing
.I check
will cause
Requesting a scrub will cause
.I md
to read every block on every device in the array, and check that the
data is consistent. For RAID1, this means checking that the copies
are identical. For RAID5 this means checking that the parity block is
correct.
data is consistent. For RAID1 and RAID10, this means checking that the copies
are identical. For RAID4, RAID5, RAID6 this means checking that the
parity block is (or blocks are) correct.
If a read error is detected during this process, the normal read-error
handling causes correct data to be found from other devices and to be
@ -458,7 +456,7 @@ If
.I repair
was used, then a mismatch will be repaired in the same way that
.I resync
repairs arrays. For RAID5 a new parity block is written. For RAID1,
repairs arrays. For RAID5/RAID6 new parity blocks are written. For RAID1/RAID10,
all but one block are overwritten with the content of that one block.
A count of mismatches is recorded in the
@ -466,19 +464,18 @@ A count of mismatches is recorded in the
file
.IR md/mismatch_cnt .
This is set to zero when a
.I check
or
.I repair
process starts and is incremented whenever a sector is
scrub starts and is incremented whenever a sector is
found that is a mismatch.
.I md
normally works in units much larger than a single sector and when it
finds a mismatch, it does not find out how many actual sectors were
affected but simply add the number of sectors in the IO unit that was
used. So a value of 128 could simply mean that a single 64K check
found an error.
finds a mismatch, it does not determin exactly how many actual sectors were
affected but simply adds the number of sectors in the IO unit that was
used. So a value of 128 could simply mean that a single 64KB check
found an error (128 x 512bytes = 64KB).
If an array is created by mdadm with
If an array is created by
.I mdadm
with
.I \-\-assume\-clean
then a subsequent check could be expected to find some mismatches.
@ -501,13 +498,13 @@ to write it out. It is quite possible that the memory will be
changed while the write-out is happening. In that case the 'clean'
flag will be found to be clear when the write completes and so the
swap subsystem will simply forget that the swapout had been attempted,
and will possibly choose an different page to write out.
and will possibly choose a different page to write out.
If the swap devices was on RAID1 (or RAID10), then the data is sent
If the swap device was on RAID1 (or RAID10), then the data is sent
from memory to a device twice (or more depending on the number of
devices in the array). So it is possible that the memory gets changed
between the two times it is sent, so different data can be written to
the devices in the array. This will be detected by
devices in the array). Thus it is possible that the memory gets changed
between the times it is sent, so different data can be written to
the different devices in the array. This will be detected by
.I check
as a mismatch. However it does not reflect any corruption as the
block where this mismatch occurs is being treated by the swap system as