On 15/01/2023 14:48, Reindl Harald wrote:
Am 15.01.23 um 15:43 schrieb Pascal Hambourg:
On 15/01/2023 at 13:13, Reindl Harald wrote:
Am 15.01.23 um 13:00 schrieb Pascal Hambourg:
Linux RAID supports TRIM/discard, but what does it do exactly ?
Does it only pass-through TRIM/discard information to the underlying
devices or can it also store information about which blocks contain
valid data in the superblock metadata?
pass-through TRIM/discard
it makes no sense to store that on the RAID layer
Wouldn't it make sense to:
- skip the initial sync at array creation
- only resync valid data areas during array resync
- reduce wear caused by useless writes on flash drives
- enable TRIM/discard with parity RAID levels by default without
relying on the underlying device capability to return zeroes on read
after TRIM
- ignore mismatches in invalid data areas when scrubbing
it would make sense but it's out of question the way mdadm is
implemented today - just store the information without a proper usecase
would make things only worser
the intentional design of mdadm is a pure RAID implementation on the
device layer with no konwledge of the filesystem on top
And that's the whole point of TRIM. It tells mdadm absolutely nothing
about the file system above APART FROM which part of the array is in use ...
everything you listed above has an answer and that's ZFS/BTRFS which has
RAID-functionality by design as part of the FS and hence knows implcit
that unused space don#t need to be mirrored
"skip the initial sync at array creation" - surely, would be nice, but
the design is "we don't know what is stord in thea RAID, our job is that
in RAID1 all mirros are identical at the phyiscal disk layer"
But what we DO know, is that when the array is *created* THERE IS
NOTHING STORED IN THE ARRAY.
TRIM is the mechanism used for one layer (any layer) to tell the layer
below what part of the block device is in use. If raid can use TRIM to
keep track of what blocks carry valid data, and what blocks contain
garbage, then the raid itself can be much more efficient and pro-active
at protecting that data.
You come over very much as believing that md-raid *should* *not* be
improved. Is it really that perfect? Some of us are looking for
opportunities to improve things, and actively supporting TRIM at the
raid level is such an obvious advance ...
Whether it's worth the effort is a different question ... :-)
Can I ask a favour. STOP POURING COLD WATER ON OTHER PEOPLES' IDEAS. It
doesn't achieve anything, and drives people away!
Cheers,
Wol
