Re: software raid and ERC

On Wed, 18 Apr 2012 11:12:57 +0800 "." [off-list ref] wrote:

Apart from the behaviour of the SCSI layer, does the linux software
raid layer have any concept of timeouts that would cause a drive to be
kicked when performing a deep recovery cycle?  A storagereview forum
thread [3] claims that the linux software raid layer does not have a
concept of timeouts and does not care about ERC.  In a web article [4]
the major NAS manufacturers that use software raid seem to agree with
this stance.

Linux software RAID does not have a concept of timeouts.

On the other hand, how I interpret a previous post from Stefan [5] is
that the linux raid layer does have its own timeout mechanism that
will kick a non-responding drive.

That aspect of that post is inaccurate.

quoted

Without ERC-timeout, the drive tries to correct the error on
its own (not reacting on any requests), mdraid assumes an error after a
while and tries to rewrite the "missing" sector (assembled from the
other disks).  But the drive will still not react to the write request
as it is still doing its internal recovery procedure.  Now mdraid
assumes the disk to be bad and kicks it.

Since I can't read code, I'm hoping that this list where software raid
development takes place would be able to clear up whether

a.  Do delays caused by deep recovery cycles actually have any direct
impact on the linux software raid layer, or does it simply issue a
command to the underlying storage/scsi subsystem and block until there
is a response?

md/raid in linux simply issues a command and waits for it to complete, either
with success or failure.

NeilBrown