On 09/02/2011 08:44, Stan Hoeppner wrote:
maurice put forth on 2/8/2011 11:37 AM:
On 2/7/2011 1:07 PM, Roberto Spadim wrote:
hi guys, could md send TRIM command to ssd? using ext4 discart
mount option? if i mix ssd and hd, could this TRIM be rewrite to
non TRIM compatible disks?
I have read that using md with SSDs is not a great idea: Form the
Fedora 14 documentation:
Using any RAID level but pure striping with SSDs is a bad idea, for
the exact reason in that documentation: excessive writes.
SSD - Solid State Drive
Note the first two words. Solid state device = integrated circuit.
ICs, including those comprised of flash memory transistors, have
totally different failure modes than spinning rust disks, SRDs, or
"plain old mechanical hard drives".
RAID'ing SSDs with any data duplicative RAID level, any mirroring or
parity RAID levels, _decreases_ the life of all SSDs in the array.
This is the opposite effect of what you want: reliability and
lifespan.
People have a misconception that SSDs are like hard disks. The only
thing they have in common is that both store data and they can have a
similar interface (SATA). The similarities end there.
RAID is not a proper method of extending the life of SSD storage nor
protecting the data on SSD devices. If you want to pool all the
capacity of multiple SSDs into a single logical device, use RAID 0 or
spanning, _not_ a mirror or parity RAID level. If you want to
protect the data, snap it to a single large SATA drive, or a D2D
backup array, and then to tape.
First off, let me agree with you that backup is important no matter what
you use as your primary storage.
But beyond that, you've got a basic assumption wrong here.
Good quality, modern SSDs do not have write-endurance issues. It's a
thing of the past. Internally, of course, the flash /does/ have
endurance limits. But these are high (especially with SLC devices
rather than MLC devices), and the combination of ECC, wear-levelling and
redundant blocks means that you can write to these devices continuously
at high speed for /years/ before endurance issues become visible by the
host. An additional effect of the extensive ECC is that undetected read
errors are much less likely than with hard disks - when a failure /does/
occur, you know it has occurred.
Many SSD models suffer from a certain amount of performance degradation
when they have been used for a while. Intel's devices were notorious
for this, though apparently they are better now. But that's a speed
issue, not a reliability or lifetime issue.
SSDs (again, I refer to good quality modern devices - earlier models had
more problems) are inherently more reliable than HDs, and have longer
expected lifetimes. This means that it is often fine to put your SSDs
in a RAID0 combination - you still have a greater reliability than you
would with a single HDD.
However, SSDs are not infallible - using redundant RAID with SSDs is a
perfectly valid setup. Obviously you will have a whole disks worth of
extra writes when you set up the RAID, and redundant writes means more
writes, but the SSDs will handle those writes perfectly well.
There is plenty of scope for md / SSD optimisation, however. Good TRIM
support is just one aspect. Other points include matching stripe sizes
to fit the geometry of the SSD, and taking advantage of the seek speeds
of SSD (this is particularly important if you are mirroring an SSD and
an HD).
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