On 29-Jan-07, at 9:04 PM, Al Hopper wrote:
> On Mon, 29 Jan 2007, Toby Thain wrote:
>
>> Hi,
>>
>> This is not exactly ZFS specific, but this still seems like a
>> fruitful place to ask.
>>
>> It occurred to me today that hot spares could sit in standby (spun
>> down) until needed (I know ATA can do this, I'm supposing SCSI
does
>> too, but I haven't looked at a spec recently). Does anybody do
this?
>> Or does everybody do this already?
>
> I don't work with enough disk storage systems to know what is the
> industry
> norm. But there are 3 broad categories of disk drive spares:
>
> a) Cold Spare. A spare where the power is not connected until
it is
> required. [1]
>
> b) Warm Spare. A spare that is active but placed into a low power
> mode. ...
>
> c) Hot Spare. A spare that is spun up and ready to accept
> read/write/position (etc) requests.
Hi Al,
Thanks for reminding me of the distinction. It seems very few
installations would actually require (c)?
>
>> Does the tub curve (chance of early life failure) imply that hot
>> spares should be burned in, instead of sitting there doing nothing
>> from new? Just like a data disk, seems to me you'd want to know
if a
>> hot spare fails while waiting to be swapped in. Do they get tested
>> periodically?
>
> The ideal scenario, as you already allude to, would be for the disk
> subsystem to initially configure the drive as a hot spare and
send it
> periodic "test" events for, say, the first 48 hours.
For some reason that's a little shorter than I had in mind, but I
take your word that that's enough burn-in for semiconductors, motors,
servos, etc.
> This would get it
> past the first segment of the "bathtub" reliability curve ...
>
> If saving power was the highest priority, then the ideal situation
> would
> be where the disk subsystem could apply/remove power to the spare
> and move
> it from warm to cold upon command.
I am surmising that it would also considerably increase the spare's
useful lifespan versus "hot" and spinning.
>
> One "trick" with disk subsystems, like ZFS that have yet to have
> the FMA
> type functionality added and which (today) provide for hot spares
> only, is
> to initially configure a pool with one (hot) spare, and then add a
> 2nd hot
> spare, based on installing a brand new device, say, 12 months
> later. And
> another spare 12 months later. What you are trying to achieve,
> with this
> strategy, is to avoid the scenario whereby mechanical systems, like
> disk
> drives, tend to "wear out" within the same general, relatively
short,
> timeframe.
>
> One (obvious) issue with this strategy, is that it may be
> impossible to
> purchase the same disk drive 12 and 24 months later. However, it's
> always
> possible to purchase a larger disk drive
...which is not guaranteed to be compatible with your storage
subsystem...!
--Toby
> and simply commit to the fact
> that the extra space provided by the newer drive will be wasted.
>
> [1] The most common example is a disk drive mounted on a carrier
> but not
> seated within the disk drive enclosure. Simple "push in" when
> required.
> ...
> Al Hopper Logical Approach Inc, Plano, TX. [EMAIL PROTECTED]
approach.com
> Voice: 972.379.2133 Fax: 972.379.2134 Timezone: US CDT
> OpenSolaris.Org Community Advisory Board (CAB) Member - Apr 2005
> OpenSolaris Governing Board (OGB) Member - Feb 2006
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