[Discuss] SSD

[blu at nedharvey.com (Edward Ned Harvey)]

> From: discuss-bounces+blu=nedharvey.com at blu.org [mailto:discuss-
> bounces+blu=nedharvey.com at blu.org] On Behalf Of Jerry Feldman
> 
> On 05/31/2012 08:03 AM, Stephen Adler wrote:
> > Hi all,
> >
> > I'm looking at upgrading my workstation by adding a couple SSDs as
> > system disks. I'm going to keep my regular drives (reconfigure them)
> > so that they are used for storage, backups etc. I've been reading some
> > reviews on newegg.com and there are a lot of postings about the drives
> > gone bad. Anyone have any advice, preference with SSDs? Any linux
> > specific advice?
>
> IMHO: I don't think it is a good idea. First, SSDs are much more costly
> per byte. AFAIK, their MTBF is much higher (eg better)  than standard
> hard drives.
> Today, I would only use an SSD in a netbook where it uses less power and
> is lighter.

That's not very helpful, and also, sort-of wrong.  Yes, SSD's are more
costly per byte than HDD's.  But so are i7 processors as compared to
whatever powers your $2 calculator.  If the price differential is small
enough and the performance (or other benefit) differential is large enough,
then you happily plunk down the extra money because the benefits outweigh
the cost.

The MTBF of SSD's is sort of a black art.  When they first came out years
ago, they posted the same MTBF, but in actuality it was much worse because
windows kept writing the same disk block over and over, which is fatal to
SSD's.  But they fixed this problem with load leveling (or wear balancing)
in hardware in the SSD, mapping virtual blocks to physical blocks.  Now they
post the same MTBF, and typically last much longer than an equivalently
rated HDD.  So it's really tough to say which is better, or how you can
know.  All I can say is, don't buy the cheapest one, and hopefully that
might help you avoid a problem down the road...

Besides lower power and lighter weight and smaller size, SSD's are much
faster in terms of IOPS.  Well...  They should be.  Even the IOPS statistics
are all a black art.  The performance out of the box is wildly different
than what it is, 6 months later.  And it depends on your OS, and it depends
on your usage patterns, etc, etc.  

So, there's no real good advice that's even conceivably possible to give
here.  (I know I'm making a big generalization, but I believe in it, this
time.)  All I can say is, try to buy a good brand, try not to buy the
cheapest one, and hopefully you'll be doing well.

Set your expectations.  You should notice IOPS improvement.  You should not
see any sequential throughput improvement.  (Perhaps even a few percent drop
in sequential throughput.)

The flash they use in USB sticks and SD cards is the same flash they use in
enterrpise hard drives.  But the controller from a USB fob, versus a
commodity SSD controller, versus an enterprise SSD controller are wildly
different, both in terms of performance and reliability.

Flash itself is dirt cheap.  What you're really buying is the on-device
flash controller, that maps the flash blocks to virtual HDD blocks, and
implements the USB/SATA operations.