database and log files on same drive

What are the risks in storing the db and logfiles on the same drive.

If its purely performance risk... why doea storing both on the same drive cause performance issues.
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pma111Asked:
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duncanb7Commented:
Database recovery is concern.
Seperate it that is better and same drive Risks reported at this site
http://support.microsoft.com/kb/2033523

and you can take  a look at relative link for reference only if have time at
http://technet.microsoft.com/en-us/library/aa996891(v=exchg.80).aspx

Duncan
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QlemoBatchelor, Developer and EE Topic AdvisorCommented:
Both reliability and performance are the reason for separating.

It does not make sense to use two partitions on the same physical disk - if it fails, both partitions are bad, and all files lost. For reliability you try to have a datafile backup and logfiles or a datafile and logfile backups available all the time.

More, neither DBMS I/O subsystem nor the OS I/O subsystem can optimize access for different partitions as good as for a single one. In general, the drive heads have to move much more in such cases.

With DBMS datafiles and logfiles use different access methods. While logfile operations tend to be more sequential and equally read/write, datafiles are read and written in a more random manner, with usually much more reads than writes.
It makes sense to use a file system adapted to those needs - and that is a different one for each.

Using RAID with redundancy (10 or 5 or similar) lessens the importance of separating files. On a RAID 10 you can place both file types into the same file system without risking performance or reliability (to a certain extend).
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Scott PletcherSenior DBACommented:
>> While logfile operations tend to be more sequential and equally read/write <<

Log file operations are overwhelmingly write, not an equal mix of read and write.  That is why RAID5 is not a good choice for log files.
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QlemoBatchelor, Developer and EE Topic AdvisorCommented:
Of course. I forgot that reading logfiles happens in memory most of the time, different from starting MSSQL.
In addition to that, logfile writes may not be buffered (need to be write-thru), and the DBMS usually forces that (MSSQL does).
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duncanb7Commented:
Thx for your pt

ve a nice day

Duncan
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