SSD? (Re: Disk I/O as a bottleneck?)

[Gilboa Davara]

On Sun, 2011-05-08 at 13:27 +0300, shimi wrote:

> 
> 
> b.t.w. IIRC when a cell dies, it does so "gracefully"; I.e. no data is
> lost, and there are spare blocks for that case... and even when
> they're all full, you just get to the point that you still have your
> data read-only. I vaguely remember I read that somewhere... and if
> it's indeed like that, this is still way better than a regular hard
> drive - those tend to usually take all your data with them, and are
> much more sensitive to many things (shock - physical/electric, heat,
> etc...)

While its very unlikely that you'll ever come close to reaching the
(more-or-less-theoretical) per-cell write limit, SSD's do have their
share of major issues that should be taken into account:


1. Bricking: As a general rule, SSD's are -far- more vulnerable to data
corruption and data loss issues and the reason for this is rather
simple:
Compared to spinning HDD's, SSD's have highly complex firmware that uses
complex write-leveling algorithms and, in some cases, complex
compression schemes. (SandForce SF-1xxx, SF-2xxx controllers)
As a result, any corner bug in the SSD's firmware will usually brick
your SSD, leaving you with nothing.
Far worse, while we (as an industry) have extensive of experience in
yanking data out of a damaged spinning HDD's, there's far less
collective experience in dealing with bricked SDD's.

2. Experience: We literally have decades of experience with dealing with
spinning HDD's. Give me a RAID with 1,000 drives and I can predict how
many drives will die each month, each year. On the other hand, we simply
do not have sufficient experience in dealing with high-end SSD's to draw
the predict the same when it comes to SSD's.

3. Performance: TRIM, file-system optimization, dealing with drives
running at >90% full capacity, etc. Getting the most out of an SSD's
requires careful planning.

Let me be clear: I do not doubt that SSD's will slowly replace spinning
drives in 90% of all the use cases. I am saying that I'd consider twice
before using SDD's in mission critical environment on in places with
inadequate backup strategy.

- Gilboa