Snapshot size?

[nextlevel]

I have a little confusion about the snapshot.

How does it works (technical)?

What size (percent) makes sense to reserve for the snapshot function?

The manual is not very informative about that topic...

Anybody who knows about?

[jamis]

From what I can tell, snapshots aren't really accessable yet on the N5200.

The primary experience I have with snapshots is on high end Network Appliance (NetApp) NAS devices. On each filesystem there is a .snapshot directory in which reside old copies of files within the filesystem that have changed. The goal would be so you can recover the version of a file from a particular date/time that has been changed, deleted or corrupted since then.

Since I use my N5200 primarily for read only operations (streaming music/video) and don't plan to have many 'changes' in files, I configured my snapshot area to be a fairly small size (4% or 76GB in my case).

I'm not sure what the optimal size would be, but I figured that was a safe amount for my needs.



Update:

I found this info that might be helpful assuming Thecus' idea of snapshots are the same as most other people's... ( www.tandbergdatacorp.com/support/support_nas_faq.htm#b2 ):

How much space should be set aside for snapshot reserved area? Back

The percentage of disk space you need to reserve for Snapshot depends on how many snapshots you will have active simultaneously, and upon the amount of change that occurs to the file and directory content of your volume while a snapshot is active. It is also correct to say that it depends on how much full-snapshot risk you are willing to accept.

The worst case would be when a source volume is entirely full and the entire content changes after taking a snapshot. This would require the snapshot to store an amount equal to the full size of the volume's original file system. If you need to allow for this worst-case behavior and you want to use seven concurrently active snapshots, 7/8 of your total disk storage should be reserved for snapshot storage.

More commonly, a user may apply administrative reasoning such as: 20% of my 80 GB volume (i.e., 16 GB) turns over daily, and I want three days worth of daily snapshots. The total storage required for this case would be estimated as 6 X 16 or 96 GB (6 being 3*4/2). If the disk space in this volume totals 176 GB, 96 GB is about 55% of the total space. This user could reserve 55% of raw disk space for Snapshot when creating the volume. But there are some additional factors to consider.

Additional Factors:

1. The size of files that are changed does not necessarily equal the size of blocks that are changed. The way block changes are handled internally is a hidden factor which can vary based on the type of file system. Unfortunately, this can make it very difficult to calculate precise disk-space requirements based on intuitive estimates of file activity.
2. Often, if 20% of the files change on the first day and 20% change on the next day, many of the changes on the second day will occur to the same blocks that changed on the first day, so two days worth of changes may not total 40%. Even at 20% changes per day, seven day's worth of changes cannot be greater than 100% and are normally much less. Thus, 55% of 176 GB in the calculation above could be more than is actually needed, and since that is more than half of the total disk space, it might be wise to cut back a little. Remember, even if you reduce the Snapshot reserve area to the point where one of your snapshots overflows occasionally, the rest of your snapshots and all of your source data will still be good.

[nextlevel]

Thank you for this information. It helps me to find the right size for the snapshot space, even if it's hard to quantify the daily data changes.

I thought, it is a less space consuming technology