Thanks! This was extremely helpful! On 12/17/2014 07:41 PM, Matthew Dillon wrote: > Swapcache will speed up accesses to the external disk very nicely, but not > have much of an effect on write performance. But writes are already > buffered to some degree, and NFS clients usually flush modifies files on > close() for safety reasons (which might be why you see some latency). That > can often be turned off on the client but I do not recommend doing so. > > The main problem with USB-attached drives is that the I/Os are basically > serialized, so random access performance is going to be horrible. For a > server you really need a SATA attached drive. Swapcache will improve > random access performance though once things get cached. > > Depending on the number of files you are storing verses the size of the > vnode cache on the server, you might want to turn on the > vfs.hammer.double_buffer feature (via sysctl). > > To use swapcache, make sure a large amount of swap space is configured on > the SSD and then enable it via sysctl. Typical starting configuration is > something like the below. 'man swapcache' for more information. > > vm.swapcache.use_chflags = 0 > vm.swapcache.read_enable=1 > vm.swapcache.meta_enable=1 > vm.swapcache.data_enable=1 > > To really be useful you want as many gigabytes of swap space as you can > bear. 100G if you can. Small amounts of swap space, like ~4GB isn't going > to do much to help cache a TB+ HDD. 40GB will help a little. The more the > better. > > -Matt > > On Wed, Dec 17, 2014 at 2:35 AM, PeerCorps Trust Fund < > [email protected]> wrote: >> >> Ha! Thanks for this. I actually set it up yesterday only to get a critical >> error notice that the external disk just couldn't keep up (my translation >> of the jargon it spit out). I had to stop the transfer and make a hard >> restart of the laptop. >> >> I am using a Thinkpad x200 and do intend on upgrading it via expresscard >> to USB 3.0. The external disk itself is USB 3.0 2 terabyte drive. >> >> FreeBSD was tested previously and works well. My draw to Dragonfly's is >> related to its NFS performance which seems to be consistently faster in my >> testing. I was also keen to experiment more with HAMMER outside of a >> virtual system. HAMMER's lower resource requirements are also a strong draw. >> >> On 12/17/2014 12:23 PM, Michael Neumann wrote: >>> >>> >>> Am 17.12.2014 um 03:52 schrieb Justin Sherrill: >>>> Short answer: yes, set up swapcache on the SSD and you'll benefit. >>>> That's the goal of swapcache. See the swapcache man page for a guide >>>> on how large to make your partition, and use the rest for / or however >>>> you want to arrange it. >>> >>> But keep in mind swapcache does not work as a write-cache. It's only >>> caching reads. >>> >>> USB 2.0 sets a limit which is higher than the 20 MB/sec you get, so I >>> assume it's your disk that is not the fastest (which I guess is quite >>> common for "USB" disks). I doubt you get more out of that disk on any >>> other operating system. >>> >>> FreeBSD/ZFS has something called ZFS Intent Log, which does write >>> caching. But I doubt you want to use it on your laptop as in general ZFS >>> is a bit more hungry in terms of resources. >>> >>> I'd suggest you to buy a good/fast harddisk and attach it via eSATA or >> USB 3.0 >>> (is it supported by DragonFly / your laptop?). Fast disks usually are >> powered >>> by an external power supply and not via the USB power (things might have >>> changed during the years). Swapcache can then help you to improve read >>> performance, which also helps write performance as it takes off read >> load from >>> the disk. A good harddisk should be able to give your 100 MB/sec of >> sequential >>> writes, but for random reads the performance is abysmal, this is where >> the SSD >>> shines. >>> >>> Regards, >>> >>> Michael >> >> >
-- Michael L. Wilson International Project Coordinator PeerCorps Trust Fund - Tanzania
