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Re: [dm-devel] [PATCH v3 0/8] dm-raid (raid456) target

On Jan 6, 2011, at 4:46 AM, NeilBrown wrote:

Patches 1 and 2 aren't really needed - I feel inclined not to remove that
stuff until all the dust settles..  So I'll put them at the bottom of my
queue and bring them forward in a couple of releases if that seems

No problem.

Patch 3 I'll push out through my tree shortly.


The rest I am happy with going out through 'dm' to -next and beyond whenever
you like.

However I will take this opportunity to suggest that the raid parameters
might need a bit of review before they are finalised.

The possible parameters are as follows:
<chunk_size> Chunk size in sectors.
[[no]sync] Force/Prevent RAID initialization
[rebuild <idx>] Rebuild the drive indicated by the index
[daemon_sleep <ms>] Time between bitmap daemon work to clear bits
[min_recovery_rate <kB/sec/disk>] Throttle RAID initialization
[max_recovery_rate <kB/sec/disk>] Throttle RAID initialization
[max_write_behind <value>] See '-write-behind=' (man mdadm)
[stripe_cache <sectors>] Stripe cache size for higher RAIDs

'rebuild idx' only allows one device to be rebuilt at a time.  With RAID6 it
is possible that two devices can be ready for rebuild, in which case we would
want to rebuild both of them.

In which case, you can specify the parameter twice:
"... rebuild 0 rebuild 1..."
No problem there, I think.

Also if you stop an array during a rebuild you really want to start again
where you were up to.

This is kept in the superblock (forthcoming patches).  In the case where metadata devices are not specified (or can't be specified, as in this initial set of patches), the user has two choices: 1) allow the array to completely resync, or 2) specify [no]sync.

Most general solution would be to combine the rebuild information with the
list of component devices - either a sector number or something to indicate
that the rebuild it complete (which could be just a v.big sector number).

Is there something more to this, or is the above comment suitable for this also?

I'd much prefer 'daemon_sleep' to be in seconds, we decimals supported if

I had this in seconds originally, but device-mapper tables already have a precedent for specifying things in ms.  We don't want to have each target pick their own units.  (A similar thing can be said for values given in sectors.  We may have wanted kiB or B, but sectors is the standard.)


3: <#raid_devs> <meta_dev1> <dev1> .. <meta_devN> <devN>

This doesn't allow offsets into the various devices like dm-stripe,
dm-linear, dm-raid1 all do.

This is a conscious choice.  The offset was discarded because it is unneeded.


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