Iodepths are 1,16,256,1024,32768 ( I know 32 or 64 should be the maximum limit, I justed wanted to try anyway).Īnd the results are almost same for all depths and for all disks (RAID 6 SSD,NVME and NFS): except for sequential read on NVME disk with 32768 depth. I have run multiple tests for each iodepth and device type, with 22 parallel jobs as the CPU count is 24 and with rwtype: sequential read and sequential write. This may happen on Linux when using libaio and not setting `direct=1', since buffered I/O is not async on that OS.ĭepth distribution in the fio output to verify that the achieved depth is as expected. If the IO is synchronous (blocking IO), we can have only one queue.Įven async engines may impose OS restrictions causing the desired depth not to be achieved. This aligns with my understanding of queue depth. Ioengines (except for small degrees when verify_async is in use). Note that increasing iodepth beyond 1 will not affect synchronous Number of I/O units to keep in flight against the file. Now, coming to the definition of iodepth in fio man page: I understand queue depth which is the number of outstanding I/O requests that the storage controller can handle ( ) i.e., this is the limitation on a storage controller which handles the I/O requests and sends the commands to disk (r/w) and it (not strictly?) drops the requests if there are more than which it can handle (which will be resubmitted by the clients presumably).Īnd the reason for having high outstading I/O requests could be multiple client connections requesting I/O or multiple processes even from a single host requesting I/O (which I thought, but it seems OS uses I/O scheduler merges the I/O requests - which are originated from buffer when doing periodic or on-demand sync and send only a fixed number of outstading requests, so that it won't overload the storage devices?)
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