This is arguably off-topic for this list, but hopefully it's relevant enough that no one gets upset... I have a conceptual question regarding "sequential" versus "random" I/O, reads in particular. Say I have a simple case: one disk and exactly one program reading one big file off the disk. Clearly, that's a sequential read operation. (And I assume that's basically a description of a sequential read disk benchmark program.) Now I have one disk with two large files on it. By "large" I mean the files are at least 2x bigger than any disk cache or system RAM, i.e. for the sake of argument, ignore caching in the system. I have exactly two programs running, and each program constantly reads and re-reads one of those two big files. >From the programs' perspective, this is clearly a sequential read. But from the disk's perspective, it to me looks at least somewhat like random I/O: for a spinning disk, the head will presumably be jumping around quite a bit to fulfill both requests at the same time. And then generalize that second example: one disk, one filesystem, with some arbitrary number of large files, and an arbitrary number of running programs, all doing sequential reads of the files. Again, looking at each program in isolation, it's a sequential read request. But at the system level, all those programs in aggregate present more of a random read I/O load... right? So if a storage system (individual disk, RAID, NAS appliance, etc) advertises X MB/s sequential read, that X is only meaningful if there is exactly one reader. Obviously I can't run two sequential read benchmarks in parallel and expect to get the same result as running one benchmark in isolation. I would expect the two parallel benchmarks to report roughly 1/2 the performance of the single instance. And as more benchmarks are run in parallel, I would expect the performance report to eventually look like the result of a random read benchmark. The motivation from this question comes from my use case, which is similar to running a bunch of sequential read benchmarks in parallel. In particular, we have a big NFS server that houses a collection of large files (average ~400 MB). The server is read-only mounted by dozens of compute nodes. Each compute node in turn runs dozens of processes that continually re-read those big files. Generally speaking, should the NFS server (including RAID subsystem) be tuned for sequential I/O or random I/O? Furthermore, how does this differ (if at all) between spinning drives and SSDs? For simplicity, assume a spinning drive and an SSD advertise the same sequential read throughput. (I know this is a stretch, but assume the advertising is honest and accurate.) The difference, though, is that the spinning disk can do 200 IOPS, but the SSD can do 10,000 IOPS... intuitively, it seems like the SSD ought to have the edge in my multi-consumer example. But, is my intuition correct? And if so, how can I quantify how much better the SSD is? Thanks, Matt -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
