2014-10-20 22:39 GMT+08:00 Wido den Hollander <wido@xxxxxxxx>: > On 10/20/2014 03:25 PM, 池信泽 wrote: >> hi, cephers: >> >> When I look into the ceph source code, I found the erasure code pool >> not support >> the random write, it only support the append write. Why? Is that random >> write of is erasure code high cost and the performance of the deep scrub is >> very poor? >> > > To modify a EC object you need to read all chunks in order to compute > the parity again. > > So that would involve a lot of reads for what might be just a very small > write. > > That's also why EC can't be used for RBD images. > But for RBD cases, the smallest write will be 4k and the largest will be 512k, which is determined by the block device driver. Currently, if we use cache-tiering, one 4k random write may promote the whole 4M object into the hot pool even though just serveral 4Ks in this object is hot. So this is unreasonable. Furthermore, for the hot pool, it can be a common replicated pool, and even, we can use kv-store for hot pool. In kv-store, the content will be stripped to 1k = 1024 byte as default. So is that possible to make use of this feature to realize the model, if miss 4k, we just promote 4k data to the hot pool and saving it into kv-store? Expecting haomai Wang's input Nicheal, Regards >> Thanks. >> >> >> >> _______________________________________________ >> ceph-users mailing list >> ceph-users@xxxxxxxxxxxxxx >> http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com >> > > > -- > Wido den Hollander > Ceph consultant and trainer > 42on B.V. > > Phone: +31 (0)20 700 9902 > Skype: contact42on > _______________________________________________ > ceph-users mailing list > ceph-users@xxxxxxxxxxxxxx > http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com _______________________________________________ ceph-users mailing list ceph-users@xxxxxxxxxxxxxx http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
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