The downside is that performance can be affected because every write request is accompanied by the need to copy out a block for retention by the snapshot. That keeps everything consistent for that snapshot, which comprises pointers to unchanged blocks and copies of those subsequently changed.Ĭopy-on-write snapshots save on space because unchanged data is referenced from the original copy (as long as not much has changed). In copy-on-write, when an I/O request seeks to change a storage block, that block is copied first and retained by the snapshot to which it belongs. There are two fundamental types of snapshot mechanism: copy-on-write and redirect-on-write.
Snapshot retention periods will usually reflect that, with deletion of snapshots occurring after, say, 48 hours, which allows for one or two backups to have taken place in that period. Snapshots are good for more frequent protection measured in minutes or hours, while backups are used for daily protection.
So, snapshots are best used in conjunction with a backup strategy. That means they are vulnerable to any outages that affect that system and they use up (potentially primary) storage capacity. The flipside to being rapidly available comes down to snapshots usually being held locally on the storage device. It is possible to set snapshots for, say, once an hour, without affecting production systems, whereas backups are likely to run once a day and outside main production hours because of their impact on resources.
Drive snapshot recovery disk plus#
Snapshots: Benefits and pitfallsĪ key benefit of snapshots is that they allow a faster roll-back to a previous point-in-time than from backups.Īnother plus is that snapshots allow much more frequent protection than backup. They don’t take up much space individually, but their total volume can grow, especially if there are lots of deleted blocks/files, and so suppliers usually limit the amount of snapshots that can be retained. So, snapshots are not backups because they are not copies. In such a case, the version rolled back to could be a mixture of metadata from previous snapshots, data retained by those snapshots and whatever counts as a parent copy (ie, before snapshots). Where blocks have been deleted, these have to be retained, and are added to the snapshot in case they are needed. In the case of rolling back, the volume or unit of storage in question would be changed to a state that reflected the snapshot, by removal and movement of blocks, etc. Snapshots are like a point-in-time copy or table of contents that shows which blocks and/or files existed and where. So, for example, you can roll back to the state of a unit of storage at any previous snapshot. They allow the user to roll back to previously existing versions of a volume, drive, file system, database, etc. Often, snapshots come as a feature of NAS or SAN storage and are created and held on that storage. Snapshots 101įundamentally, a snapshot is a record – comprised of metadata – that indicates the state of blocks and files in a unit of storage. We will also briefly look at replication and see how that fits with snapshots and backup.
In this article, we will look at snapshots and to what extent they can be a replacement or an addition to backup.
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