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: >> 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. -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
