On Sunday August 23, for.poige+linux@gmail.com wrote:
2009/8/23 NeilBrown [off-list ref]:
[...]
quoted
Yes it will be updated. But if it is updated with a
read-modify-write cycle then an old incorrect value will be updated
to a new incorrect value, which doesn't help you much.
I see. Thank you for the answer. My assumption was due thinking that
parity is never being used when RAID5 is in "healthy" mode.
But anyway, please clear it up: if I've made --asume-clean RAID5
array, formatted it with some FS (or even LVM), and put OS/data to it,
then (in idle at night, for e. g.) run "resync", would that be the
same valid result (overall data integrity) as if I had previously
created it without "--asume-clean" at all? (Let's suppose hard disks
were healthy in both cases :-)
If by 'run "resync"' you mean
echo repair > /sys/block/mdXX/md/sync_action
then yes, what will have the same effect as not using --assume-clean
to start with.
And Neil, using the chance to have your reply :-) I'd like to ask one
more question: is that true (I think and hope it is!) that MD doesn't
read/write full chunk size when it's just to read/write another block
of data and hence chunk size only affects data interleaving on disks.
That is correct - chunk size only affect data layout, not IO size.
md/raid5 always reads or writes a whole page (normally 4K) at a time.
NeilBrown
P. S. Is there someone on this list who can share his linux-RAID
archive in mbox format? From time to time I find interesting
discussions have been going but can't just hit "Reply". If you can
help me with that, please reply personally (not CC:ing to the list).
Thanks!
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On Wed, Aug 26, 2009 at 2:25 AM, Neil Brown [off-list ref] wrote:
On Sunday August 23, for.poige+linux@gmail.com wrote:
quoted
2009/8/23 NeilBrown [off-list ref]:
[...]
quoted
Yes it will be updated. But if it is updated with a
read-modify-write cycle then an old incorrect value will be updated
to a new incorrect value, which doesn't help you much.
I see. Thank you for the answer. My assumption was due thinking that
parity is never being used when RAID5 is in "healthy" mode.
But anyway, please clear it up: if I've made --asume-clean RAID5
array, formatted it with some FS (or even LVM), and put OS/data to it,
then (in idle at night, for e. g.) run "resync", would that be the
same valid result (overall data integrity) as if I had previously
created it without "--asume-clean" at all? (Let's suppose hard disks
were healthy in both cases :-)
If by 'run "resync"' you mean
echo repair > /sys/block/mdXX/md/sync_action
then yes, what will have the same effect as not using --assume-clean
to start with.
quoted
And Neil, using the chance to have your reply :-) I'd like to ask one
more question: is that true (I think and hope it is!) that MD doesn't
read/write full chunk size when it's just to read/write another block
of data and hence chunk size only affects data interleaving on disks.
That is correct - chunk size only affect data layout, not IO size.
md/raid5 always reads or writes a whole page (normally 4K) at a time.
NeilBrown
Hmm...so how read from some program -> fs -> md happens then? I.e. -
if FS has stride/stripe/swidth for, say, 64kb (and md chunk size is
64kb), some program requests 128kb of data to be read, does it mean
we'll do 16 reads for 4kb from two drives? Even if we do, is it
right/good ?
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[COOLCOLD-RIPN]
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On Wed, August 26, 2009 9:34 am, CoolCold wrote:
On Wed, Aug 26, 2009 at 2:25 AM, Neil Brown [off-list ref] wrote:
quoted
On Sunday August 23, for.poige+linux@gmail.com wrote:
quoted
2009/8/23 NeilBrown [off-list ref]:
That is correct - chunk size only affect data layout, not IO size.
md/raid5 always reads or writes a whole page (normally 4K) at a time.
NeilBrown
Hmm...so how read from some program -> fs -> md happens then? I.e. -
if FS has stride/stripe/swidth for, say, 64kb (and md chunk size is
64kb), some program requests 128kb of data to be read, does it mean
we'll do 16 reads for 4kb from two drives? Even if we do, is it
right/good ?
Actually it is a bit more complicated than that.
Md/raid5 tries to encourage the filesystem to submit read requests is sizes
that fit inside one chunk. When md/raid5 gets such a read request,
and it is for a device that has not failed and no reshape is happening
(and maybe some other conditions), it will forward the read request
down to the drive that can serve it.
If any of these conditions aren't met, then the read request will
be split in to 4K chunks and each chunk will be handled separately,
possibly reconstructing from other devices if that is needed etc.
However if a number of consecutive 4K reads (or writes) are submitted
to a device, the driver for that device will normally assemble
those requests together into fewer larger requests so you shouldn't
get lots of little request going out to the device unless there
are lots of little requests coming down from the filesystem.
NeilBrown