On Tue, 12 Sep 2006, Anton B. Rang wrote:
.... reformatted ....
> >True - I'm a laptop user myself. But as I said, I'd assume the whole disk
> >would fail (it does in my experience).
Usually a laptop disk suffers a mechanical failure - and the failure rate
is a lot higher than disks in a fixed location environment.
> That's usually the case, but single-block failures can occur as well.
> They're rare (check the "uncorrectable bit error rate" specifications)
> but if they happen to hit a critical file, they're painful.
>
> On the other hand, multiple copies seems (to me) like a really expensive
> way to deal with this. ZFS is already using relatively large blocks, so
> it could add an erasure code on top of them and have far less storage
> overhead. If the assumed problem is multi-block failures in one area of
> the disk, I'd wonder how common this failure mode is; in my experience,
> multi-block failures are generally due to the head having touched the
> platter, in which case the whole drive will shortly fail. (In any case,
The following is based on dated knowledge from personal experience and I
can't say if its (still) accurate information today.
Drive failures in a localized area are generally caused by the heads being
positioned in the same (general) cylinder position for long periods of
time. The heads ride on a air bearing - but there is still a lot of
friction caused by the movement of air under the heads. This is turn
generates heat. Localized heat buildup can cause some of the material
coated on the disk to break free. The drive is designed for this
eventuality - since it is equipped with a very fine filter which will
catch and trap anything that breaks free and the airflow is designed to
constantly circulate the air through the filter. However, some of the
material might get trapped between the head and the disk and possibly
stick to the disk. In this case, the neighbouring disk cylinders in this
general area will probably be damaged and, if enough material accumulates,
so might the head(s).
In the old days people wrote their own head "floater" programs - to ensure
that the head was moved randomly across the disk surface from time to
time.
I don't know if this is still relevant today - since the amount of
firmware a disk drive executes, continues to increase every day. But in a
typical usage scenario, where a user does, for example, a find operation
in a home directory - and the directory caches are not sized large
enough, there is a good probability that the heads will end up in the same
general area of the disk, after the find op completes. Assuming that the
box has enough memory, the disk may not be accessed again for a long time
- and possibly only during another find op (wash, rinse, repeat).
Continuing: a buildup of heat in a localized cylinder area, will cause the
disk platter to expand and shift, relative to the heads. The disk
platter has one surface dedicated to storing servo information - and from
this the disk can "decide" that it is on the wrong cylinder after a head
movement. In which case the drive will recalibrate itself (thermal
recalibration) and store a table of offsets for different cylinder ranges.
So when the head it told, for example, to move to cylinder 1000, the
correction table will tell it to move to where physical cylinder 1000
should be and then add the correction delta (plus or minus) for that
cylinder range to figure out where to the actually move the heads to.
Now the heads are positioned on the correct cylinder and should be
centered on it. If the drive gets a bad CRC after reading a cylinder it
can use the CRC to correct the data or it can command that the data be
re-read, until a correctable read is obtained. Last I heard, the number
of retries is of the order of 100 to 200 or more(??). So this will be
noticable - since 100 reads will require 100 revolutions of the disk.
Retries like this will probably continue to provide correctable data to
the user and the disk drive will ignore the fact that there is an area of
disk where retries are constantly required. This is what Steve Gibson
picked up on for his SpinRite product. If he runs code that can determine
that CRC corrections or re-reads are required to retrieve good data, then
he "knows" this is a likely area of the disk to fail in the (possibly
near) future. So he relocates the data in this area, marks the area
"bad", and the drive avoids it. Given what I wrote earlier, that there
could be some physical damage in this general area - having the heads
avoid it is a Good Thing.
So the question is, how relevant is storing multiple copies of data on a
disk in terms of the mechanics of modern disk drive failure modes.
Without some "SpinRite" like functionality in the code, the drive will
continue to access the deteriorating disk cylinders, now a localized
failure, and eventually it will deteriorate further and cause enough
material to break free to take out the head(s). At which time the drive
is toast.
> multi-block failures could be addressed by spreading the data from a
> large block and using an erasure code.)
Regards,
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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