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]
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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