RAID 5 write performance


Ad

Advertisements

I

Igor Batinic

Hi!
Since it's striped I would expect RAID 5 to be faster than RAID 1 generally,
but I've read some vague references (in this link and elsewhere) to lower
write performance in 5 than in 1. Why would that be so?

http://www.msexchange.org/articles/Transaction-Logs-Lifeblood-Exchange.html

Indeed, RAID 5 (usually) is much slower that RAID 1 in such cases,
especially if you use RAID 5 array for such a purpose without a lot of
cache, battery backup etc.

RAID 5 is not recommended array for transaction logs not only in
Exchange environment, but also in all other environments.

Best regards,

Iggy
 
A

Arno Wagner

Previously Tom Del Rosso said:
Since it's striped I would expect RAID 5 to be faster than RAID 1 generally,
but I've read some vague references (in this link and elsewhere) to lower
write performance in 5 than in 1. Why would that be so?

Unkess you rite exactly in stripe size, in RAID5 you have to
read the rest of the stripe from the other drives, before
writing (to do the checksum). With RAID1 you just write,
regardless of size. This is mostly an issue for small accesses.

Arno
 
T

Tom Del Rosso

Igor Batinic said:
Hi!
http://www.msexchange.org/articles/Transaction-Logs-Lifeblood-Exchange.html

Indeed, RAID 5 (usually) is much slower that RAID 1 in such cases,
especially if you use RAID 5 array for such a purpose without a lot of
cache, battery backup etc.

RAID 5 is not recommended array for transaction logs not only in
Exchange environment, but also in all other environments.

But why is it slower in write mode? Does the controller write the parity
multiple times even when all the drives are modified?
 
T

Tom Del Rosso

Arno Wagner said:
Unkess you rite exactly in stripe size, in RAID5 you have to
read the rest of the stripe from the other drives, before
writing (to do the checksum). With RAID1 you just write,
regardless of size. This is mostly an issue for small accesses.

Thanks.
 
I

Igor Batinic

Hi!
But why is it slower in write mode? Does the controller write the parity
multiple times even when all the drives are modified?

Transaction (usually) are small records, and also usually each
transaction is written only to one disk in array. RAID 5 is most
efficient when "full stroke write" is usual.
That means: if small record is written to only one or two disks in
array, worst case scenario is to read data from all other disks,
calculate parity and then new data must be written to a couple disks in
array.
Therefore RAID 5 is not recommended array for any intensive I/O operation.

Best regards,

Iggy
 
Ad

Advertisements

C

calypso

Tom Del Rosso said:
Since it's striped I would expect RAID 5 to be faster than RAID 1 generally,
but I've read some vague references (in this link and elsewhere) to lower
write performance in 5 than in 1. Why would that be so?

http://www.cs.berkeley.edu/~pattrsn/252F96/Lecture13.pdf

From one of the guys that invented RAID... Page 11 iz what you want to
see...


--
Mercedesa slusa u saboru plav jabukao udise danas. By runf

Damir Lukic, [email protected]_MAKNIOVO_fly.srk.fer.hr
http://inovator.blog.hr
http://calypso-innovations.blogspot.com/
 
Ad

Advertisements


Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments. After that, you can post your question and our members will help you out.

Ask a Question

Top