On 02/05/11 19:39, Peter Kovari wrote:
Hi all,
I understand, that a change from RAID5 to RAID6 by adding a single disk
-
eg. keeping the number of data disks - requires a backup file throughout
the
whole reshape process. For a larger, multi-TB array this means millions
of
writes to the backup file, which - if i'm correct - means means millions
of
writes to the same physical sectors of the disk that holds the backup
file.
Is this not problematic? How many write operations can a typical drive
tolerate nowadays? (on the same sectors)
Lots, where Lots>= 1 and Lots< infinity.
I've never seen rotating media specify any form of limitation to writes.
Have you?
No, that's why i'm asking.
Imho, in typical usage, write cycle counts on a certain sector may not be
that high, even on a database server. I doubt it ever goes over a few
hundred thousands during the life cycle of the hard disk. On the other hand,
a single reshape on a larger array can trigger tens of millions of write
cycles on certain sectors. Sectors do fail eventually, so I'm wondering if
the "no limit" is truly a no limit, or manufacturers just won't state this
info because in "normal" usage, customers will never reach that limit.
Btw, i'm sure SSD's are not meant to take such a pressure.
Good large SSD's can be written to continuously for /years/ before they
wear out. It can be a different matter for smaller and cheapo drives,
but it's not an issue for good disks now. Suppose you have a 128 GB
disk with SLC flash. Each sector is good for roughly 100,000
erase/re-write cycles (or more, if you are kind to the disk and keep it
cool). Since wear-leveling spreads the writes around the disk, you can
write 100,000 x 128 GB of data - at 200 MB/s continuously, that would
take 2 years without a pause for breath. Even if the wear-leveling
isn't perfect, and even if you substitute a cheaper MLC SSD (with 10,000
cycles), the effort of being the backup file for a raid reshape is not
going to be a challenge.
Also, some SSD's have super-cap backed up ram caches - writes can be
safely buffered before being written. If you overwrite the same sector
fast enough, it will never actually be written to the flash (until the
final write, of course).
For hard disks, sectors do wear out, but they tolerate a lot of writes
first. And the hard disk firmware will re-locate the worn out sector
transparently.
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