Re: software raid and ERC

On 04/17/2012 10:05 AM, . wrote:

I'm trying to decide what disks to use for a software raid array that
will host mirrors of open source stuff.  So this array will run 24/7
and resiliency to disk failures is needed, but service levels can be
similar to a home file server.  ie, it's ok if one of the disks goes
into a deep recovery cycle for a few minutes once a month and I can't
host the stuff - people will just retry the download.  Backups aren't
really important either, as I can just mirror all the content again
(even if it takes weeks to do so).

Due to budget reasons, "enterprise" disks are out. What I've read
strongly recommends the ERC / TLER / CCTL feature for raid
applications - even including software raid.  But is ERC really
required in my scenario?

The Wikipedia article at
http://en.wikipedia.org/wiki/Error_recovery_control#Software_Raid on
ERC seems to suggest that mdadm will not error out the drive no matter
how long the recovery takes.  Instead, the SCSI disk layer is the
limiting factor, as a lengthy recovery cycle could lead to a scsi
command timeout, ignoring the drive reset command, and leading to the
disk being marked offline.  If this is indeed the case, I am tempted
to just set the scsi timeout value to 5 minutes (or whatever the
maximum period that deep recovery can take).  Are there other similar
timeouts or gotchas in other layers?  Eg, in LVM, FS code, etc?

I've been burned by this very phenomenon on a set of Seagate drives
that I thought had SCTERC, but didn't (their predecessors did).

I wasn't aware that the driver timeouts were configurable.  Pointers?

In another post
(http://marc.info/?l=linux-raid&m=130964222812107&w=2), Drew said:

quoted

TLER just shortens the firmware's error recovery from something like
60 seconds down to 4 seconds. It's mainly useful in hardware RAID but
I can see it being useful with mdraid in the enterprise where you
can't afford to wait for the drive to do it's own recovery attempts.

In my use case, I really don't mind if the server freezes for a while.

Please advise if there are other considerations for ERC, use of
consumer-grade disks, or "enterprise" disks.  Thanks!

Be aware that SCTERC must be set on any drive power cycle--it's not a
persistent setting on desktop drives.

P.S. I would buy ERC if I could, but the right hard disk models do not
seem to be available locally.  My preference is for the Hitachi 7k3000
series, but it seems to be out of stock with possibly months of delay.
 So what's left are the consumer 2TB models with ERC feature -
possibly just the Hitachi 2TB or Samsung Spinpoint F4 2TB.  I'd
appreciate any other suggestions too.

The 7k3000 family is my preference at the moment.  Fortunately I don't
have an immediate need.