SSDs

[D. Hugh Redelmeier]

| From: Ted <ted.leslie-Re5JQEeQqe8AvxtiuMwx3w at public.gmane.org>

| I first used a intel high-end  SLC, no issues.
| I then used a MLC (because cheaper). No issues with data loss, but i sometimes
| get unexplained pauses that make me think its gone off and
| doing bit leveling (algo in device). The SLC i think i rated for 100k w
| cycles?, MLC 1000-10,000 IRC?, so i am wondering if going to a MLC was
| mistake, having said that, 99% of them now seem to be MLC's.

In (my) theory, this is an important issue: SLC vs MLC.  SLC's have so
much longer lifetime but they are mostly driven out of the (consumer?)
market.

SLC should (in theory) be twice the price per byte as MLC: as I
understand it, mainstream MLC cells store two bits (there is work on
increasing this) whereas SLC cells store one bit (by definition).

If you use SSD purely as cache, common sense says that it gets
hammered by writes.  So SLC makes sense for this application.  That, I
presume, is why Seagate uses SLC for its hybrid drives.

| ... Ram and SSD flash ram seemed about same price(SLC),
| again making ram disk a viable alternative for some systems.

If SLC is as expensive as RAM, why not use RAM?

A cache that is stable (in the face of power failures and other system
crashes) is much more effective than one that is not, all other things
being equal.

The classic example is databases: when a transaction is committed, it
must reach stable storage.  The log could be held in SSD or disk,
but not conventional RAM (unconventional works: battery backed-up
hardware RAM-Disk).

The same idea applies to many less formalised settings.  Obviously
file-system operations are an example.


So: how should we Linux users use mass-market SSDs?

- Consider desktops or other PC systems that are not space or weight
  constrained.  Mass-market (MLC) SSDs are 2.5" (usually) and are
  cheap, large, and fast enough to hold filesystems.  I use one for /
  (which I can reconstruct if it goes south) but not /home (it's on a
  conventional hard drive).  I also like lots of disk space, something
  that only hard drives can provide economically.

- I think that the Seagate hybrid drives are quite attractive for
  notebooks.  They look just like an ordinary 2.5" hard drive but have
  a transparent but effective SLC SSD cache.

- for many notebooks, SSDs make sense: 2.5" size makes them direct
  replacements for notebook hard drives.  Some notebooks even have
  1.8" slots which really are best for SSDs.  Often notebooks don't
  need the bulk capacity of hard drives.  (120G SSD costs roughly
  the same as a 1T hard drive.)

- I hesitate to use MLC SSDs as ordinary caches.  I do feed my systems
  lots of RAM for that.

- some notebooks support mSATA drives in addition to their 2.5" drive
  slot.  mSATA SSDs are more expensive and slower than SATA ones but
  I think that they could be worthwhile.  I'm seriously thinking
  of adding one to my new ThinkPad T520.


I installed a 120G SSD on my desktop a few months back.  I use it for
/, not home.  My impressions:

- I only notice a few speed improvements:

  + booting is faster, but I rarely boot, so it isn't a big win

  + software updates are a lot faster ("yum update" in Fedora).
    I think that the reason is that these operations are
    random-access intensive.  Still, I don't spend a lot of time
    waiting upon Linux software update.

- I'm pretty sure I'd notice a lot of slowdowns if I switched back.
  That's just human nature.

The way I use the SSD, 30G would probably be enough.  As I get braver,
I might put more on it.  My current desktop use doesn't really hammer
the disk.

SSDs could be quite useful for databases.  It depends on the actual
application of the databases.  Lennart gave a good example, MythTV's
database (which does not include the actual recording files): I find
my Myth TV database is a bit slow on regular hard drives.
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