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@xxxxxxxxx> 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? 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@xxxxxxxxxxxx > https://lists.01.org/mailman/listinfo/linux-nvdimm > -- 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>