Patrick O'Callaghan wrote: > Has anyone actually compared the speed of pendrives versus hard disks > when used for swap? Aaron Konstam wrote: > They have to be much slower … I don’t think they have to be – I suspect they might be faster than hard disks. What I’ve read is that the sort of flash you tend to get in these drives can be very slow for writes, slow to read back data in sequentially, but they can be a lot faster than hard drives to actually start accessing the data, because there’s no physical movement required to start reading from the disk. A quick google resulted in http://www.tomshardware.com/reviews/security-flash-storage,1804-6.html , with a couple of drives with access times a tenth of what you’d expect from a good hard drive, but slower sequential transfer speeds. That leaves two questions – does the flash in question have low access times, or did the manufacturer come up with slow flash because no-one cared? And what sort of access patterns do you get for reading from swap? A virtual memory system works in pages, normally of 4K¹. If you’re just going to read in one 4K page, then the access time is going to be much larger than the transfer time, even for flash: in other words, the flash will have got the data into RAM before the hard drive is likely to be anywhere near the data. So for a hard disk to be faster than low-access-time flash for swap, then you’re going to have to have a situation where the kernel wrote out around a megabyte of data to swap, and then read it back in continuously. I doubt that Linux (or many other operating systems) does that. It will normally choose pages to swap from multiple different processes, and in any case, adjacent pages in swap would be unlikely to be needed back in RAM at the same time. Note that the speed of writing pages to swap is not that important until it takes a large proportion of the flash’s time (it’s asynchronous: nothing’s waiting on it), or unless you suddenly have a lot of need for memory (which is rare: the kernel does try to keep some free memory about, and will only actually allocate memory when a process actually starts using it). As I and others have said, just because it’s fast doesn’t mean it’s a good idea, and you might get through flash drives fairly quickly if you actually use your swap for much. (If you just have swap for emergencies, it might see less use than your home directory.) James. ¹ Other sizes may be used on other OSes, or in Linux on non-x86-compatible processors. -- E-mail: james@ | And the cuckoo isn’t cooing, aprilcottage.co.uk | But he’s cucking and he’s ooing, | And a Pooh is simply pooh-ing | Like a bird. -- ‘Noise’, by Pooh -- fedora-list mailing list fedora-list@xxxxxxxxxx To unsubscribe: https://www.redhat.com/mailman/listinfo/fedora-list Guidelines: http://fedoraproject.org/wiki/Communicate/MailingListGuidelines
