Boaz Harrosh <boaz <at> plexistor.com> writes: > > On 02/26/2016 12:04 PM, Thanumalayan Sankaranarayana Pillai wrote: > > On Thu, Feb 25, 2016 at 10:02 PM, Dan Williams <dan.j.williams <at> intel.com> wrote: > >> [ adding Thanu ] > >> > >>> Very few applications actually care about atomic sector writes. > >>> Databases are probably the only class of application that really do > >>> care about both single sector and multi-sector atomic write > >>> behaviour, and many of them can be configured to assume single > >>> sector writes can be torn. > >>> > >>> Torn user data writes have always been possible, and so pmem does > >>> not introduce any new semantics that applications have to handle. > >>> > > > > I know about BTT and DAX only at a conceptual level and hence do not understand > > this mailing thread fully. But I can provide examples of important applications > > expecting atomicity at a 512B or a smaller granularity. Here is a list: > > > > (1) LMDB [1] that Dan mentioned, which expects "linear writes" (i.e., don't > > need atomicity, but need the first byte to be written before the second byte) > > > > (2) PostgreSQL expects atomicity [2] > > > > (3) SQLite depends on linear writes [3] (we were unable to find these > > dependencies during our testing, however). Also, PSOW in SQLite is not relevant > > to this discussion as I understand it; PSOW deals with corruption of data > > *around* the actual written bytes. > > > > (4) We found that ZooKeeper depends on atomicity during our testing, but we did > > not contact the ZooKeeper developers about this. Some details in our paper [4]. > > > > It is tempting to assume that applications do not use the concept of disk > > sectors and deal with only file-system blocks (which are not atomic in > > practice), and take measures to deal with the non-atomic file-system blocks. > > But, in reality, applications seem to assume that 512B (more or less) sectors > > are atomic or linear, and build their consistency mechanisms around that. > > > > This all discussion is a shock to me. where were these guys hiding, under a rock? > > In the NFS world you can get not torn sectors but torn words. You may have > reorder of writes, you may have data holes the all deal. Until you get back > a successful sync nothing is guarantied. It is not only a client > crash but also a network breach, and so on. So you never know what can happen. > > So are you saying all these applications do not run on NFS? Speaking for LMDB: LMDB is entirely dependent on mmap, and the coherence of a unified buffer cache. None of this is supported on NFS, so NFS has never been a concern for us. We explicitly document that LMDB cannot be used over NFS. Speaking more generally, you're talking nonsense. NFS by default transmits *pages* over UDP - datagrams are all-or-nothing, you can't get torn words. Likewise, NFS over TCP means individual pages are transmitted with individual bytes in order within a page. > Thanks > Boaz > > > [1] http://www.openldap.org/list~s/openldap-devel/201410/msg00004.html > > [2] http://www.postgresql.org/docs/9.5/static/wal-internals.html , "To deal > > with the case where pg_control is corrupt" ... > > [3] https://www.sqlite.org/atomiccommit.html , "SQLite does always assume that > > a sector write is linear" ... > > [4] http://research.cs.wisc.edu/wind/Publications/alice-osdi14.pdf > > > > Regards, > > Thanu > > _______________________________________________ > > Linux-nvdimm mailing list > > Linux-nvdimm <at> lists.01.org > > https://lists.01.org/mailman/listinfo/linux-nvdimm > > -- -- Howard Chu CTO, Symas Corp. http://www.symas.com Director, Highland Sun http://highlandsun.com/hyc/ Chief Architect, OpenLDAP http://www.openldap.org/project/ -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>