Neil Brown wrote:
On Monday January 26, James.Bottomley@xxxxxxxxxxxxxxxxxxxxx wrote:
On Mon, 2009-01-26 at 12:34 -0500, Greg Freemyer wrote:
Adding mdraid list:
Top post as a recap for mdraid list (redundantly at end of email if
anyone wants to respond to any of this).:
== Start RECAP
With proposed spec changes for both T10 and T13 a new "unmap" or
"trim" command is proposed respectively. The linux kernel is
implementing this as a sector discard and will be called by various
file systems as they delete data files. Ext4 will be one of the first
to support this. (At least via out of kernel patches.)
SCSI - see http://www.t10.org/cgi-bin/ac.pl?t=d&f=08-356r5.pdf
ATA - see T13/e08137r2 draft
Per the proposed spec changes, the underlying SSD device can
optionally modify the unmapped data. SCSI T10 at least restricts the
way the modification happens, but data modification of unmapped data
is still definitely allowed for both classes of SSD.
Thus if a filesystem "discards" a sector, the contents of the sector
can change and thus parity values are no longer meaningful for the
stripe.
This isn't correct. The implementation is via bio and request discard
flags. linux raid as a bio->bio mapping entity can choose to drop or
implement the discard flag (by default it will be dropped unless the
raid layer is modified).
That's good. I would be worried if they could slip through without
md/raid noticing.
ie. If the unmap-ed blocks don't exactly correlate with the Raid-5 / 6
stripping, then the integrity of a stripe containing both mapped and
unmapped data is lost.
Thus it seems that either the filesystem will have to understand the
raid 5 / 6 stripping / chunking setup and ensure it never issues a
discard command unless an entire stripe is being discarded. Or that
the raid implementation must must snoop the discard commands and take
appropriate actions.
No. It only works if the discard is supported all the way through the
stack to the controller and device ... any point in the stack can drop
the discard. It's also theoretically possible that any layer could
accumulate them as well (i.e. up to stripe size for raid).
Accumulating them in the raid level would probably be awkward.
It was my understanding that filesystems would (try to) send the
largest possible 'discard' covering any surrounding blocks that had
already been discarded. Then e.g. raid5 could just round down any
discard request to an aligned number of complete stripes and just
discard those. i.e. have all the accumulation done in the filesystem.
To be able to safely discard stripes, raid5 would need to remember
which stripes were discarded so that it could be sure to write out the
whole stripe when updating any block on it, thus ensuring that parity
will be correct again and will remain correct.
Probably the only practical data structure for this would be a bitmap
similar to the current write-intent bitmap.
Is it really worth supporting this in raid5? Are the sorts of
devices that will benefit from 'discard' requests likely to be used
inside an md/raid5 array I wonder....
raid1 and raid10 are much easier to handle, so supporting 'discard'
there certainly makes sense.
NeilBrown
--
The benefit is also seen by SSD devices (T13) and high end arrays
(T10). On the array end, they almost universally do RAID support
internally.
I suppose that people might make RAID5 devices out of SSD's locally, but
it is probably not an immediate priority....
ric
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