Brenda Price wrote:
> We are looking at a new server for our future needs (approximately 2nd
> quarter 2008) and Dell is recommending a RAID 6.
[ ... snipped ... ]
> We currently have RAID 1+0.
>
In terms of performance RAID-6 (and RAID-5) are bad ideas for database
servers. Database servers do a lot of random writes. Assuming 15K
SCSI/SAS drives, you can get about 250 random 4K writes/sec from a
single bare drive. With Raid-10, you get about 250 per drive pair.
With Raid-5/6 you get about 250 unless you have enough controller memory
to cache strip misses whereas you will get a bit better than 250.
Large arrays of hard disks are always Raid-10. Keep a hot-spare in the
rack if you are worried. Some people even "extra mirror" Raid-10. For
example, the Linux software raid driver lets you setup Raid-10 with 2,
3, or even 4 mirrored copies of the data. Even stranger, you can setup
a 5 drive Raid-10 array with 2 copies of each data block, but I digress.
In any case, don't let anyone sell you a hard drive array for an active,
updating, database that is Raid-5 or Raid-6 running on rotating hard
drives. If this is what they are selling you, then they did not hear
the word "database".
If you are looking for speed, your options are:
* Lots of 15K drives Raid-10
* Some sort of new solid state storage
[ADV] My apologies for the ad mixed in.
I just finished some Linux benchmarks on an 8-drive Raid-5 setup on
2.6.22 64-bit Linux. This is a combination of very fast Mtron flash
SSDs plus the "Managed Flash Technology" driver that I developed. This
is the first time I was able to get my hands on 8 Mtron drives at the
same time.
You should note that what I said about databases and Raid-5 does not
apply to solid state drives. With SSDs, and particularly when the
Managed Flash driver is in use, Raid-5 works very well.
Here is a part of the array's performance table:
Random Read Tests:
Block 1 thread | 10 threads | 40 threads
Size IO/sec Bandwidth | IO/sec Bandwidth | IO/sec Bandwidth
| |
4K 9528.7 37.22MBps | 40972.2 160.04MBps | 50980.7 199.14MBps
8K 7286.5 56.92MBps | 33799.5 264.05MBps | 40663.0 317.67MBps
16K 4821.9 75.34MBps | 24169.1 377.64MBps | 30628.7 478.57MBps
256K 1188.4 297.09MBps | 2846.8 711.69MBps | 3096.6 774.14MBps
2M 250.5 501.00MBps | 395.9 791.79MBps
Random Write Tests:
Block 1 thread | 10 threads | 40 threads
Size IO/sec Bandwidth | IO/sec Bandwidth | IO/sec Bandwidth
| |
4K 37696.3 147.25MBps | 47256.7 184.59MBps | 46542.8 181.80MBps
8K 24011.0 187.58MBps | 27360.8 213.75MBps | 27031.9 211.18MBps
16K 12680.7 198.13MBps | 13899.7 217.18MBps | 13517.5 211.21MBps
256K 820.3 205.07MBps | 951.9 237.97MBps | 905.6 226.39MBps
2M 109.4 218.79MBps | 108.6 217.19MBps
With 64GB drives, this array would yield about 400GB of usable space.
Adding this "drive kit" to an existing server would be about $24K, which
is a lot more than hard disks would cost. But then again, at 40K to 50K
random IOPS (IOs Per Second), this array can sustain the same
transaction rate as about 200 15K drives. Even better, single-threaded
batch jobs can run 40x faster.
[/ADV]
Have fun with Dell. Make sure they sell you what you need, not what
they want to sell.
Doug Dumitru
EasyCo LLC
http://managedflash.com for more information on our solid state storage
arrays.
>
>
> Thanks all!
>
>
>
> Brenda L Price
>
> Senior Programmer Analyst
>
> Affiliated Acceptance Corporation
>
> Sunrise Beach, MO
>
> (800)233-8483
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