Re: Intel SSDs that may not suck

2011/3/29 Jeff <threshar(at)torgo(dot)978(dot)org>:
>
> On Mar 29, 2011, at 12:13 AM, Merlin Moncure wrote:
>
>>
>> My own experience with MLC drives is that write cycle expectations are
>> more or less as advertised. They do go down (hard), and have to be
>> monitored. If you are writing a lot of data this can get pretty
>> expensive although the cost dynamics are getting better and better for
>> flash. I have no idea what would be precisely prudent, but maybe some
>> good monitoring tools and phased obsolescence at around 80% duty cycle
>> might not be a bad starting point.  With hard drives, you can kinda
>> wait for em to pop and swap em in -- this is NOT a good idea for flash
>> raid volumes.
>
>
>
> we've been running some of our DB's on SSD's (x25m's, we also have a pair of
> x25e's in another box we use for some super hot tables).  They have been in
> production for well over a year (in some cases, nearly a couple years) under
> heavy load.
>
> We're currently being bit in the ass by performance degradation and we're
> working out plans to remedy the situation.  One box has 8 x25m's in a R10
> behind a P400 controller.  First, the p400 is not that powerful and we've
> run experiments with newer (p812) controllers that have been generally
> positive.   The main symptom we've been seeing is write stalls.  Writing
> will go, then come to a complete halt for 0.5-2 seconds, then resume.   The
> fix we're going to do is replace each drive in order with the rebuild
> occuring between each.  Then we do a security erase to reset the drive back
> to completely empty (including the "spare" blocks kept around for writes).
>
> Now that all sounds awful and horrible until you get to overall performance,
> especially with reads - you are looking at 20k random reads per second with
> a few disks.  Adding in writes does kick it down a noch, but you're still
> looking at 10k+ iops. That is the current trade off.
>
> In general, i wouldn't recommend the cciss stuff with SSD's at this time
> because it makes some things such as security erase, smart and other things
> near impossible. (performance seems ok though) We've got some tests planned
> seeing what we can do with an Areca controller and some ssds to see how it
> goes.
>
> Also note that there is a funky interaction with an MSA70 and SSDs. they do
> not work together. (I'm not sure if HP's official branded ssd's have the
> same issue).
>
> The write degradation could probably be monitored looking at svctime from
> sar. We may be implementing that in the near future to detect when this
> creeps up again.

svctime is untrustable. From the systat author, this field will be
removed in a future version.

--
Cédric Villemain               2ndQuadrant
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