Posts Tagged ‘append-on-write’

Explain Snapshots

March 11th, 2014

This seems to be a popular search term so I think it’s worth covering off. This is covered on my old top post about Fractional Reservation, but I’ll cover the alternatives here also.


NetApp snapshots used to be pretty unique in the industry, but the industry term for this technology is generally now Append-on-Write / Redirect-on-Write (new writes are appended to the “end”, or redirected to free blocks, depending how you look at it) and quite a few vendors do it this way. Put very simply, all new data is written to new (zeroed) blocks on disk. This does mean that snapshot space has to be logically in the same location as the production data, but that really shouldn’t be a problem with wide-striping / aggregates / storage pools (pick preferred vendor term). When a snapshot is taken, the inode table is frozen and copied. The inode table points to the data blocks, and these data blocks now become fixed. As the active filesystem “changes” blocks, these actually get written to new locations on disk, and so there is no overhead to the write (the new blocks are already zeroed). In other technologies (not NetApp) this also forms the basis of automated tiering, once data is “locked” by a snapshot, it’ll never be over-written so it can safely be tiered out of SSD or even SAS as read performance is rarely an issue. NetApp use FlashPools to augment this, and a snapshot is a trigger for data to be tiered out of FlashPools as it’ll never be “overwritten”.

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