SAN volumes and high water marks

[ArizonaGeek]

We have a 4tb Compellent SAN that we are using for storage. I have a few volumes created on it and one of them is a 2tb volume I was using for some storage as well as our users "My Documents" and it filled up pretty quickly. Well, at least 1tb of the data was just crap we are required to keep for compliance reasons and it was never going to change so we bought some attached storage, moved the data off our SAN and cleared up over 1tb off that one volume. Total I have cleared up around 2 to 2.5tb of space on all of our volumes but the 2tb is the biggest volume.

Well this morning I came in and found our SAN on conservation mode because it was running out of space. I called Compellent and they are telling me Microsoft has a high water mark, when the volume is used it won't give back the space when it is cleared. Since we used 1.8tb of a 2tb volume at one point, which now only has 419gb of data, Compellent is saying that Windows isn't releasing the now unused space. They say there is some high water mark in Windows that wont release blocks once it has been used. I can't find any information on it doing Google searches or searching the Microsoft KB.

Anyone else know about this high water mark? Are they BS'ing me & I am just not finding the info? If this is true how can I get that space back without deleting my volume and recreating it? The data that is there now is stuff that I just can't afford downtime to move. I figured a defrag would give me that back so I am using Raxco Perfect Disk to do weekly defrags but they are saying it won't give it back.

Thanks for any info!

Cheers
Rob

The answer is always "PEBKAC!

 

[RMGBELGIUM]

They are not BS-ing you.Windows does not reuse space on a volume that resides on a virtualized SAN until it believes it has reached the end of the volume - then it starts over and uses space that is marked free at the beginning of the volume.
Try running a defrag would as well be my suggestion.Some storage systems apperently have a problem with this,and unfortunately I haven't come by a solution yet.

rgds,

R.

 

[ChawdogISC]

Compellent has a great article on the partner portal titled "Dealing with Thin Provisioning and High Water Marks (v2)Last updated 8/15/07"
This document will answer all your questions. But this part I believe is what you are looking for...
Windows built in Disk Defragmenter does not reorder space on a Windows Volume. As of 8/15/2007,
Compellent recommends the use of Diskeeper Defragmentation Software. When Diskeeper is
run, deleted blocks will once again be available. Windows will then write over previously deleted
blocks instead of writing to new ones. This will prevent the high water mark from being moved
further down the volume and make sure that Storage Center is not using more space then it
needs to.

 

[nhidalgo]

We had the same problem with our compellent SAN, Diskeeper works to a point, but will increase the size of your replays until the old data expires. The best solutions we found for now was to create a temp volume, copy the data off, deleted the replays and erase the data from you production LUN and then copy the data back. I heard there is a new shrink process in 2008 server which is supposed to take care of this problem.

Nick

 

[ArizonaGeek]

Nick,

Pretty much that was what I was left to do except the volume was 2TB and not enough space on our system to create a smaller volume I needed.

What I wound up doing was backing the data up to tape, deleting the volume and recreating it smaller then dumping it back from tape. Luckily I did it over a 3 day weekend (this thread is a couple of months old) and had plenty of time to make sure I got it right. Had I know about this problem I would have started off with smaller volumes to separate out the data but we were given wrong information.

I have heard of other SANs offering a solution to the high water mark where the SAN takes back control of the volume from Windows and forces the unused space back to the SAN. Off the top of my head I cant think of the ones I saw that did it but I wish Compellent offered this a few months ago. The support and sales guys all tell us its coming but they want to keeps us happy so I think they'll say anything as long as we keep our support payments.

Cheers
Rob

The answer is always "PEBKAC!

 

[ChawdogISC]

Well as a partner with them I would be interested in knowing what sales/eng person that you talked to told you that. Last year at C-Drive I was told that from the engineers that write it. Of course personally I would rather them work on the replication piece.