Re: raid1 - ssd, doubts

[Andrei Banu <andrei.banu@redhost.ro>]

Hi,

I am in no way of the same technical caliber as many of the people on this
list so please take my advice with a grain of salt. However I do have some
experience with Samsung SSDs in RAID 1 software (md-raid) and I would like
to warn you against it.

My setup is with 840 PROs of 512GB. I did leave free space (not partitioned)
for OP. The SSDs are connected on SATA3.

I'll give you the result of 2 tests and if you are happy, go ahead:

READ
root [~]# hdparm -t /dev/sda
/dev/sda:
  Timing buffered disk reads: 292 MB in  3.01 seconds =  96.92 MB/sec

WRITE
root [~]# dd if=/dev/zero of=testn bs=4k count=256k conv=fdatasync
262144+0 records in
262144+0 records out
1073741824 bytes (1.1 GB) copied, 13.4047 s, 80.1 MB/s

When the setup is fresh you'll get significantly more out of it but 
after a while
(this setup is roughly 1 year old) this is what you get with Samsung SSDs in
RAID1 SW. At least this is my experience and I did try to improve it. As 
a matter
of fact, before making a secure erase of one of the SSDs the write speed 
was
under 10MB/s. What you see above is a lot better than what it started out
(but it's true that after the secure erase the read/write speeds were a lot
better but it dropped in a very short time frame).

A few points:
1. CAUTION: 840 EVO are proved to have a read speed degradation for old
data written to the drive (this is unrelated to RAID but it probably 
affects
performance in RAID).
2. I believe that any SSDs, regardless of the brand, in a software 
RAID-1 setup
might be a bad idea.
3. I believe that the same SSDs in a hardware RAID setup might lead to a
different story.

Again: I am not a technical wiz (like many of the people on this list) 
but I did
have some experience with this so I thought I should let you know.

Kind regards!



On 30.09.2014 17:50, Roberto Spadim wrote:
> hi guys!
> i will use a ssd raid1, i want know if raid1 trim is supported at mdadm
> i will use a 840 evo (or evo pro not selected the right one yet) 500gb
> each, raid1, today database size is 100gb, i think it will grow
> 10gb/year, i had many space...
>
> the point are: madm raid1 trim is supported? or should i use lvm?
> should i partition it with 400gb and leave 100gb untouched? or should
> i use a hdd+ssd and dmcache?
>
>
> :) thanks guys, that's a small enterprise solution, they can't buy
> raid cards and sas harddisk are same price of ssd :)
>
> any idea/experience and information is wellcome
>
>