> On Jun 6, 2017, at 10:35 AM, Webert de Souza Lima <webert.b...@gmail.com> > wrote: > > I'd like to add that, from all tests I did, the writing of new files only go > directly to the cache tier if you set hit set count = 0. > >
Is there any concern or disadvantage once I set hit set count = 0? Looks like it is used to record the read/write recency only? Sorry for the stupid question but i’m trying to understand the cache-tier behavior :) Thanks, Ting Yi Lin > Em Seg, 5 de jun de 2017 23:26, TYLin <wooer...@gmail.com > <mailto:wooer...@gmail.com>> escreveu: >> On Jun 5, 2017, at 6:47 PM, Christian Balzer <ch...@gol.com >> <mailto:ch...@gol.com>> wrote: >> >> Personally I avoid odd numbered releases, but my needs for stability >> and low update frequency seem to be far off the scale for "normal" Ceph >> users. >> >> W/o precise numbers of files and the size of your SSDs (which type?) it is >> hard to say, but you're likely to be better off just having all metadata >> on an SSD pool instead of cache-tiering. >> 800MB/s sounds about right for your network and cluster in general (no >> telling for sure w/o SSD/HDD details of course). >> >> As I pointed out before and will try to explain again below, that speed >> difference, while pretty daunting, isn't all that surprising. >> > > SSD: Intel S3520 240GB > HDD: WDC WD5003ABYZ-011FA0 500GB > fio: bs=4m iodepth=32 > dd: bs=4m > The test file is 20GB. > >> No, not quite. Re-read what I wrote, there's a difference between RADOS >> object creation and actual data (contents). >> >> The devs or other people with more code familiarity will correct me, but >> essentially as I understand it this happens when a new RADOS object gets >> created in conjunction with a cache-tier: >> >> 1. Client (cephfs, rbd, whatever) talks to the cache-tier and the >> transaction causes a new object to be created. >> Since the tier is an overlay of the actual backing storage, the object >> (but not necessarily the curent data in it) needs to exist on both. >> 2. Object gets created on backing storage which involves creating the >> file (at zero length), any needed directories above and the entry in the >> OMAP leveldb. All on HDDs, all slow. >> I'm pretty sure this needs to be done and finished before the object is >> usable, no journals to speed this up. >> 3. Cache-tier pseudo-promotes the new object (it is empty after all) and >> starts accepting writes. >> >> This is leaving out any metadata stuff CephFS needs to do for new "blocks" >> and files, which may also be more involved than overwrites. >> >> Christian > > You make it clear to me! thanks! Really appreciate your kind explanation. > > Thanks, > Ting Yi Lin > _______________________________________________ > ceph-users mailing list > ceph-users@lists.ceph.com <mailto:ceph-users@lists.ceph.com> > http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com > <http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com>
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