Re: [pbs-devel] [PATCH proxmox-backup 2/2] tape: use datastores 'read-thread' for tape backup

[Dominik Csapak <d.csapak@proxmox.com>]

On 4/30/24 11:39, Dominik Csapak wrote:
> using a single thread for reading is not optimal in some cases, e.g.
> when the underlying storage can handle more reads in parallel than
> with a single thread.
> 
> This depends largely on the storage and cpu.
> 
> We use the ParallelHandler to handle the actual reads.
> Make the sync_channel buffer size depending on the number of threads
> so we have space for two chunks per thread.
> 
> Did some benchmarks on my (virtual) pbs with a real tape drive (lto8
> tape in an lto9 drive):
> 
> For my NVME datastore it did not matter much how many threads were used
> so i guess the bottleneck was either in the hba/drive or cable rather
> than the disks/cpu. (Always got around ~300MB/s from the task log)
> 
> For a datastore on a single HDD, the results are much more interesting:
> 
> 1 Thread:  ~55MB/s
> 2 Threads: ~70MB/s
> 4 Threads: ~80MB/s
> 8 Threads: ~95MB/s
> 
> So the fact that multiple IO request are done in parallel does speed up
> the tape backup in general.
> 

eh that sentence might be misleading, what i meant was not 'in general'
but for the case of spinning disks

could be amended before applying or in a v2


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