Offline defragmentation
in [Microsoft Exchange]
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From: C''est bien moi on 14 Apr 2010 14:14 Hello, Is that the article, below, from Microsoft is true even if the disks are on a SAN ? I ask that because of the word "computer" in the following sentence: "The disk defragmentation process rearranges the data that is stored on the computer's hard disks so that the files are more contiguous" http://support.microsoft.com/kb/328804/en-us What I understood is with a physical disk that maintains 64 sectors per track, Windows always creates the partition starting at the sixty-forth sector, therefore misaligning it with the underlying physical disk. To be certain of disk alignment, we use "Diskpart" to partition tool. Exchange Server 2003 writes data in multiples of 4 KB I/O operations (4 KB for the databases). Therefore, the starting offset is a multiple of 4 KB. Failure to do so may cause a single I/O operation spanning two tracks, causing performance degradation. This means that a single I/O operation does not spanned on 2 tracks. During the defragmentation copies database records to a new database. The disk defragmentation process rearranges the data so that the files are more contiguous. Thanks in advance for the reply.
From: M on 15 Apr 2010 11:09 Hello: The best thing is to check with your SAN vendor about that. Each SAN is implemented differently and abstracts the underlying disks so you never really access the "raw" disks. The MS article is very general and cannot possibly cover ever different disk or SAN configuration. Also, what you mentioned about sector alignment might not be necessary because the SAN presents the disks as LUNs, so you need to be careful with doing that on a SAN. Again, check with your SAN vendor. My suggestion to any sys admin who uses a SAN is to check with your SAN vendor first and use the SAN vendor's best practices. These articles from MS and others, unless specifically stated, assume that you're using direct attached disks, so you can't take everything and apply it directly to a SAN. -- Regards, M MCTS, MCSA http://SysAdmin-E.com "C''est bien moi" <Cestbienmoi(a)discussions.microsoft.com> wrote in message news:40AD3807-C281-4C57-9131-53A4D7741A9D(a)microsoft.com... > Hello, > > Is that the article, below, from Microsoft is true even if the disks are > on > a SAN ? I ask that because of the word "computer" in the following > sentence: > "The disk defragmentation process rearranges the data that is stored on > the > computer's hard disks so that the files are more contiguous" > > http://support.microsoft.com/kb/328804/en-us > > What I understood is with a physical disk that maintains 64 sectors per > track, Windows always creates the partition starting at the sixty-forth > sector, therefore misaligning it with the underlying physical disk. To be > certain of disk alignment, we use "Diskpart" to partition tool. > Exchange Server 2003 writes data in multiples of 4 KB I/O operations (4 KB > for the databases). Therefore, the starting offset is a multiple of 4 KB. > Failure to do so may cause a single I/O operation spanning two tracks, > causing performance degradation. > This means that a single I/O operation does not spanned on 2 tracks. > During > the defragmentation copies database records to a new database. The disk > defragmentation process rearranges the data so that the files are more > contiguous. > > Thanks in advance for the reply.
From: Mark Arnold [MVP] on 15 Apr 2010 18:52 On Thu, 15 Apr 2010 11:09:20 -0400, "M" <m(a)nowhere.com> wrote: >Hello: > >The best thing is to check with your SAN vendor about that. Each SAN is >implemented differently and abstracts the underlying disks so you never >really access the "raw" disks. The MS article is very general and cannot >possibly cover ever different disk or SAN configuration. Also, what you >mentioned about sector alignment might not be necessary because the SAN >presents the disks as LUNs, so you need to be careful with doing that on a >SAN. Again, check with your SAN vendor. > >My suggestion to any sys admin who uses a SAN is to check with your SAN >vendor first and use the SAN vendor's best practices. These articles from MS >and others, unless specifically stated, assume that you're using direct >attached disks, so you can't take everything and apply it directly to a SAN. Correct. Almost certainly your SAN team will go ballistic the second you mention it to them. Microsoft suggest you don't do it in the first place and move all the mailboxes to a new store instead. If you try and do the offline defrag on SAN based disk or disk that is protected by (say) Doubletake or MS DPM for that matter, you're going to touch blocks. Touching blocks will be catastrophic for your snapshots. So essentially the advice is that it's not a good idea to do under any circumstances for 2007/2010 dbs because they're protected using VSS and a very bad idea for a 2003 environment on a SAN.
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