Re: SQL Server for Oracle DBAs
Date: Thu, 29 May 2008 19:24:47 +0100
"Mark Townsend" <markbtownsend_at_sbcglobal.net> wrote in message news:483E3BA3.4040400_at_sbcglobal.net...
> > in SQL Server we don't do it that way
>> - we
>> put the logs on their own mirrored pair; we put the data on it's own RAID
>> array etc...
> Out of curiosity - why ?
The LDF files (Transaction log) is sequentially written to; it therefore is good to have the disk head at the place it last wrote; also - you don't want a mix of read/write traffic on the array where the LDF's are held - the Transaction Log is SQL Server's weakness in terms of high levels of scalability when dealing with large numbers of small transaction insert/update/deletes.
Tables can be put on seperate file groups; it's a method of balancing read and write load within the database; those file groups can sit on their own dedicated raid arrays and the IO isolated. It also allows scenario's where one file group can read at max speed because it's not got disk heads bouncing reading and writing and whilst reading from that file group - writing out to the other - again, you can achieve the max sustained write speed because the disk heads aren't bouncing around doing read and writes.
There is plenty of research out there; there is a really good article on MS recommending DASD over SAN's too - basically how they scale MSN and hotmail.
I'm suprised oracle recommend's SAME because I'd have thought at the file IO level that if you are reading a 30GB table and writing out 20GB the ideology behind it is the same whatever product is doing it.
-- Tony Rogerson, SQL Server MVP http://sqlblogcasts.com/blogs/tonyrogerson [Ramblings from the field from a SQL consultant] http://sqlserverfaq.com [UK SQL User Community]Received on Thu May 29 2008 - 13:24:47 CDT