Migrating to a SAN Environment – Do You Still Need to Defragment?
clustered/SAN file system to pool disk arrays into a virtualized storage
volume. This is not NTFS, but rather proprietary software, provided by a SAN
hardware or software vendor such as EMC (Celerra), NetApp, LSI (StoreAge
SVM), VMware (VMFS), etc. VMFS, for example, uses between 1MB to 8MB storage
blocks. This file system essentially “runs on top of NTFS”, it does not
replace it. Keeping in mind that every file system is a “virtual” disk,
Stacking one virtual component over another (i.e. one file system on top of
another) is very doable and increasingly more common.
(Photo: http://www.newscom.com/cgi-bin/prnh/20110316/443549 )
What the vendor of a SAN file system does at “that” file system is
irrelevant to what a decent third party defragmenter does. Claims that “you
do not need to defragment” may be misunderstood and incorrectly implied to
mean “NTFS”. It is very possible that you do not need to defragment the “SAN
file system”. The experts on that file system and the source from which you
should get setup tips, best practices, and SAN I/O optimization methodologies
is that manufacturer.
As for NTFS, it still fragments and causes the Windows OS to “split” I/O
requests for files sent into the SAN, creating a performance penalty. You can
measure this using Window’s built-in PerfMon tool and watch the split I/O
counter. You can also use the Average Queued Disk I/O, given you account for
the number of physical spindles. Given that SANs are ONLY ever block-level
storage, they do NOT know what I/Os relate to what files. Therefore they
cannot intelligently spread the fragments of a file across multiple disks. A
whole mass of separate I/Os writes/reads for fragmented files (which will
most certainly be interspersed with other simultaneous data writes/reads)
will be non-optimally spread across the disks in the SAN storage pool (i.e.
write more fragments of a given file to one disk rather than evenly spreading
the data across all the disks).
SAN file system vendors may offer optimization strategies to, over time,
move data around the disks as it learns typical data requests (such as from
that fragmented file incorrectly laid out on the disks) are not properly
load-balanced across SAN spindles. Generally speaking, the above holds true
for disk striping as well (RAID). SAN designers or developers agree that NTFS
fragmentation IS an issue and advanced defragmentation is important (“basic”
defragmenters can actually cause problems).
Also when the SAN starts getting performance hits caused by excessive
I/O’s resulting due to the fragmentation within NTFS, a common way in which
companies improve SAN performance, particularly I/O performance, is to simply
add more disks. In general we are masking the problem by adding more
disks/spindles to spread the excessive I/O’s, but the actual problem still
remains (excessive I/O’s would still be generated as the NTFS still
fragments) and you will end up purchasing additional hardware. While SAN
manufacturers would certainly not discourage this, it might not be the
optimal solution. Handling the fragmentation within the NTFS is the only way
to get rid of the actual problem.
Diskeeper(R) data performance software is a fully automatic defragmenter
and is fully “proactive” on the basis that it prevents fragmentation from
occurring before it happens. Diskeeper prevents up to 85% fragmentation
before it occurs, which leaves little fragmentation to handle afterwards.
http://www.diskeeper.com
Windows IT Pro analyst
covers the issues file fragmentation causes on SAN storage, entitled
“Maximize the Performance of your Windows SAN Infrastructure”. To download
click here (http://downloads.diskeeper.com/pdf/Performance-Windows-SAN.pdf)
SOURCE Diskeeper Corporation