Unnecessarily Large EqualLogic Snapshots


Unnecessarily Large EqualLogic Snapshots

by Brad Fair

EqualLogic’s snapshot functionality effectively freezes the data on the volume, and writes any changes off to another part of the disk. This excellent method of snapshots ensures that you have access to the entire volume as it was right when the snapshot was created, while at the same time minimizing the disk space required for each snapshot. That said, there are a couple things happening under the hood in your OS and software that can make your snapshots larger than needed.

The Problem – My EqualLogic Snapshots are TOO BIG!

Like I mentioned above, the only data (there’s an exception, but its minimal) that takes up space for each snapshot is the difference, at the block level, of the volume. This means when files change, file attributes change, or data gets moved / written / rewritten, it will take up space in the snapshot. Following that logic, if your snapshots are too big, there’s something causing changes to the volume. *cue scary music*

Last Access Attribute

Windows keeps track of the last time a file was touched by updating a timestamp called the Last Accessed Timestamp. This could be handy if you really have a need to know when the files were last accessed, but it’s detrimental to storage space on your EqualLogic array. Once that timestamp is updated, that portion of the disk is changed, resulting in an increased snapshot size.

The Solution – Fixing the Last Access Issue

This one’s an easy one. Disable the use of that attribute! Check it out: http://technet.microsoft.com/en-us/library/cc758569(WS.10).aspx

Change that, reboot, and watch your snapshot size decrease!

Backups and the Archive Attribute

Many backup programs use the Archive attribute to determine whether a file or folder has been changed since the last backup. The exact same concept as above, once that archive bit changes, it backs up that portion of the disk, taking up more space for your snapshots.

The Solution – Fixing the Archive Bit Issue

If your backup software supports it, use a database to keep track of what files have been backed up before. That way, when the database changes, its all the same part of the disk, and the files that are spread out over the volume remain unchanged.

Disk Defragmentation

This one is a pretty easy one – when you defragment a disk, you’re touching and altering every last block of the disk (well, at least the ones with data on them). If you have automatic defragmentation jobs scheduled, you’ll want to be careful about whether you actually get any benefits from it. In all reality, it could be that defragging groups commonly changed files together on the disk, making your snapshot size smaller afterwards. It might be worth it to try on occasion, but remember that every time you defrag, the next snapshot will be gigantic.

Summing It Up

There are several reasons your snapshots might be larger than you expect. Most of these reasons are that some piece of software is altering an attribute as it works with files. Keep an eye on your OS, Backup Software, AntiVirus software, and anything else that does file/directory scans to see if they’re the cause of the issue. If you have any other tips, I’d be happy to hear them!

More About the Author

Brad Fair

Solutions Architect
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