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NTFS allocation size

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Will an allocation size of 64kb lead to better backup and validation speed over the default 4kb?

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I do not think that speed of backup or validation would be effected by allocation size. 

When the application is setup to perform a full disk backup that backup is run at the block level meaning that full blocks on disk are backed up as complete chunks of data.  File/folders do not follow this rule.

TI 2020 has introduced enhanced usage of metadata for backups.  How extensive or to what degree this usage of metadata is applied is unknown.  Acronis continues to strive to improve the speed at which the application works.  Much of the speed factor is hardware dependent as well so the better and faster the hardware the faster the app performs.

I think that a lot of that would have to do with what your dataset is.  If you have lots of little small files, then a 64kb block size will be detrimental to backup speed.  conversely, if you have lots of large files that comprise the dataset, then yes, a larger block size will assist, but not really from a backup perspective, but more from an overall speed perspective.  It's why on database servers and email servers that we use a larger block size on the NTFS volumes.  For a general NTFS workload, the default block size is best and offers a good compromise between slack space (non-full blocks that are used up, causing some unused space to never be used), and performance. CBT or Changed Block Tracking will always yield the best performance from a backup utility, which is why incremental schemes are the fastest.

So a .tibx file isn't actually a single hundred-gb file but instead a collection of the original source files?

Essentially, it is a wrapper of that data - like a .zip file or any other compressed file.  Yes, it ends up being one large file (or whatever size file limit you set that could be in smaller chunks), but the data within it still exists as the original content).  You're limitations in performance are going to be writing that data into the larger .tib as well as reading the data from the original source.  I really don't think the difference will be much, if any at all.  The biggest change in performance comes from the type of write media and the capabilities it possesses for 4K read/write.  Beyond that, the allocation size really isn't going to make a lot of difference.

To really find out though, do some trial and error with the original allocation size - at least 2 or 3 times to get an average.  Then repeat the same backups with the larger allocation size and see if there is any noticeable increase in performance.  My guess, if using the exact same setup / disks, it really isn't going to amount to much difference, if any at all.