On Sun, 2009-02-22 at 00:21 +0000, James Wilkinson wrote: > 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.) You're forgetting that USB transfers consume a fair amount of cpu time, unlike a SATA disk which is DMA. That's why I asked if anyone had actually done a comparison. With so many complex interactions between paging policy, system load, disk configurations, wide variations in flash memory speeds etc., theory is a poor substitute for actual measurement. poc -- fedora-list mailing list fedora-list@xxxxxxxxxx To unsubscribe: https://www.redhat.com/mailman/listinfo/fedora-list Guidelines: http://fedoraproject.org/wiki/Communicate/MailingListGuidelines
