On Fri, 16 Sep 2011, Liyun Yu wrote:
Thanks David, it is good to know this.
One of our team members suggested that the SSD also tend to be
wear out every 12-16 months. I was puzzled by that statement since
I never know if the RAM or other chip-based memory device would be
wear out electronically. That is another topic maybe.
Primer on SSD flash technology limits
all of the chip based memory devices that are able to maintain the data
without power have a limited number of write cycles.
In the case of modern flash drives this is in the millions of cycles on an
individual block.
older flash drives wear out faster than newer ones, MLC flash drives wear
out faster than SLC drives (but MLC stores two bits per cell nstead of one
and has further price advantages due to volume production so it's _far_
cheaper)
flash drives write data in chunks much larger than disk sectors (128K per
chunk is common and far from the largest), if the disk doesn't buffer
writes agressivly there is a symptom called 'write magnification' that
makes it so that when you write one 512 byte block of data to the disk it
is forced to write 128K of data to the disk. As far as I know, all modern
drives have large ram buffers to limit this effect.
erasing data on a flash drive is very slow, SSDs get around this by
writing a new copy of the data to a new location and erasing the old copy
sometime later. All modern drives contain more flash than what you pay for
so that they have a large pool of 'blank' space that they can write to
(this also helps spread out the writes across more flash so that you are
less likely to hit the 1m write limit on any one spot). If you are doing a
LOT of writing to the SSD you can run into the situation where the drive
no longer has blank spots to write the data to and it has to slow down to
erase space before it can write any more. you can reduce the likelyhood of
running into this by not using all the space on the disk so it has more
unused space (also by picking a drive, controller card, and OS that
support the TRIM command so that when you free up space on disk you can
tell the drive that you don't care about it amymore and it doesn't have to
preserve it)
most flash drives are slower to write to than a high performance spinning
drive, but they eliminate seek overhead so they can still be a win when
writing, and can be a HUGE win for random reads.
I purchased a couple dozen machines with the Intel x25E SLC flash drives
about three years ago. these machines are log servers and so writing data
continuously (hundreds of gigs per day) I have had a half dozen of them
'fail' on me, and from discussions on the postrgres performance list this
is not an uncommon thing. In most cases the failure seems to be firmware,
not storage failures, but the bottom line is they quit working. In theory
when a drive wears out you should still be able to read the last data
written to it, you just won't be able to write anything. In practice there
are times when you just can't access the disk.
David Lang
Thanks,
Liyun
On 9/16/11 2:07 PM, [email protected] wrote:
On Fri, 16 Sep 2011, Liyun Yu wrote:
Anyone used the OWC (other world computing) SSD (6G) for RAID Array?
Any suggestions/recommendations on SSD based RAID on the market
or drives good for RAID ? Or any pointers to some reference benchmark?
Yes I do have budget issues. I also visited SSd page on Tom's hardware
site
which was more than 8 months old.
I did not access the SSD market for a while. Yesterday one of our
colleagues
sent me a link at OWC which indicated that their drives has writing speed
above 500MB
for some of their 6G SSD drives. I thought SSD was at 150MB usually
therefore
this is very attractive to me for our app which pulls a high volume of
dataflow from
the central RAID.
one thing to watch out for with SSDs, most of them do not have good
behavior in the face of power failures. they all cache writes in ram on the
disk and most of them have no way of getting those writes to flash if the
power fails.
David Lang
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