Обсуждение: PG over NFS
Hi all, This has been discussed before (some URLs below), but the threads have unfortunately been rather free of (precise) information. I am interested in getting PG running over NFS. However, I am primarily concerned with safety/recoverability (on sudden power loss); I care very, very little about the performance. Hence, I think this is a substantially simpler question to answer definitively (must also assume the NFS server may lose power). The particular NFS client and server implementations I'm using are the Linux NFS implementation (using kernel 2.6). If PG is unsuitable for this task, can any (preferrably open-source) alternatives be recommended? (Just for curiosity, consider any storage system supporting transactions and recovery, not necessarily the relational model or high performance.) BTW I've included some correspondence from my colleagues; I would also appreciate it if any corrections are offered to their statements (if necessary). From querying #postgresql on FreeNode, I gathered that as long as fsync works properly (flushes data to the server), there are no other concerns (and that there is in fact no file locking, except perhaps on the pid file). Thanks a lot! Yang http://archives.postgresql.org/pgsql-performance/2004-06/msg00215.php http://www.thescripts.com/forum/thread422520.html http://lists.freebsd.org/pipermail/freebsd-database/2006-January/000372.html > Hmm... that's bad. The memory mapped IO we can live without (sure, > it's a performance issue, but at least it'll probably still work). > Mutexes and locks... I wonder if we could put them in a ramdrive? > > > It is not recommended to place database storage onto NFS. This is > > seems to be true for Postgresql, SQLite, and BerkleyDB, it seems that > > this is a general problem. The reasons are: > > * DBs use memory mapped IO > > * DBs use mutexes and locks > > > > Presumably, neither of these work over NFS. I don't have any hard > > evidence, I just read a bunch of FAQs and messages on mailings lists. > > If this is indeed the case, that pretty much kills our plan for > > hosting a DB on the slave. The "no,no" alternative is to place the DB > > server on the master... and that would only work out of the box for > > non-embedded databases.
There is GFS2, OCFS, DRBD, ENBD, iSCSI, AoE and a ton of other technologies. What on earth is the point in trying to use a DBMS over NFS? :) In case it's just for the fun of it, maybe consider: - davfs2 - curlftpfs > However, I am primarily concerned with safety/recoverability (on sudden power loss); Well then.. forget about NFS :) What about various replication solutions like slony, 8.2 warm standby log shipping, mammoth replicator? > must also assume the NFS server may lose power A raid controller with battery backed cache and/or an UPS might be a good start. If that's not an option disable all write caches or use a filesystem that supports write barriers. Yang wrote: > This has been discussed before (some URLs below), but the threads have > unfortunately been rather free of (precise) information. I am > interested in getting PG running over NFS. However, I am primarily > concerned with safety/recoverability (on sudden power loss); I care > very, very little about the performance. Hence, I think this is a > substantially simpler question to answer definitively (must also > assume the NFS server may lose power). The particular NFS client and > server implementations I'm using are the Linux NFS implementation > (using kernel 2.6). > > If PG is unsuitable for this task, can any (preferrably open-source) > alternatives be recommended? (Just for curiosity, consider any storage > system supporting transactions and recovery, not necessarily the > relational model or high performance.) > > BTW I've included some correspondence from my colleagues; I would also > appreciate it if any corrections are offered to their statements (if > necessary). From querying #postgresql on FreeNode, I gathered that as > long as fsync works properly (flushes data to the server), there are > no other concerns (and that there is in fact no file locking, except > perhaps on the pid file). -- Best regards, Hannes Dorbath
Yang wrote: > This has been discussed before (some URLs below), but the threads > have unfortunately been rather free of (precise) information. I am > interested in getting PG running over NFS. However, I am primarily > concerned with safety/recoverability (on sudden power loss); I care > very, very little about the performance. In my experience PG over NFS works fine, except that the kernel will occasionally lock up under load. But that is a general NFS problem. -- Peter Eisentraut http://developer.postgresql.org/~petere/
On 3/26/07, Hannes Dorbath light-at-theendofthetunnel.de |postgresql| <...> wrote: > There is GFS2, OCFS, DRBD, ENBD, iSCSI, AoE and a ton of other > technologies. What on earth is the point in trying to use a DBMS over > NFS? :) > > In case it's just for the fun of it, maybe consider: > - davfs2 > - curlftpfs > > > However, I am primarily concerned with safety/recoverability (on sudden power loss); > > Well then.. forget about NFS :) Could you offer any explanation as to why? > What about various replication solutions > like slony, 8.2 warm standby log shipping, mammoth replicator? The environments involve two small devices - one with a flash disk (the NFS server), and a slave which network-boots off that. Hence these suggestions don't address the problem. (Would all the alternative protocols listed at the top be able to coexist with the described environment? Both devices must be able to boot into Linux.) > > > must also assume the NFS server may lose power > > A raid controller with battery backed cache and/or an UPS might be a > good start. If that's not an option disable all write caches or use a > filesystem that supports write barriers. On 3/26/07, Peter Eisentraut peter_e-at-gmx.net |postgresql| <...> wrote: > Yang wrote: > > This has been discussed before (some URLs below), but the threads > > have unfortunately been rather free of (precise) information. I am > > interested in getting PG running over NFS. However, I am primarily > > concerned with safety/recoverability (on sudden power loss); I care > > very, very little about the performance. > > In my experience PG over NFS works fine, except that the kernel will > occasionally lock up under load. But that is a general NFS problem. We've also been using PG over NFS but had not yet been concerned with reliability in the face of failures. Thanks, Yang
On 3/26/07, A.M. agentm-at-themactionfaction.com |postgresql| <...> wrote: > > On Mar 26, 2007, at 19:29 , Yang wrote: > > > On 3/26/07, Hannes Dorbath light-at-theendofthetunnel.de |postgresql| > > <...> wrote: > >> There is GFS2, OCFS, DRBD, ENBD, iSCSI, AoE and a ton of other > >> technologies. What on earth is the point in trying to use a DBMS over > >> NFS? :) > >> > >> In case it's just for the fun of it, maybe consider: > >> - davfs2 > >> - curlftpfs > >> > >> > However, I am primarily concerned with safety/recoverability (on > >> sudden power loss); > >> > >> Well then.. forget about NFS :) > > > > Could you offer any explanation as to why? > > > >> What about various replication solutions > >> like slony, 8.2 warm standby log shipping, mammoth replicator? > > > > The environments involve two small devices - one with a flash disk > > (the NFS server), and a slave which network-boots off that. Hence > > these suggestions don't address the problem. (Would all the > > alternative protocols listed at the top be able to coexist with the > > described environment? Both devices must be able to boot into Linux.) > > Since you're booting from the NFS server, it would make more sense to > have your boot process start a postgresql instance from a copy of the > data directory instead of over NFS, no? Certainly, that way, you can > have multiple instances booted and running. Do you need to sync back > to the NFS server? The second device has no non-volatile storage. (Sorry I should've explicitly stated this.) Yang > > Cheers, > M >
"Yang" <jkfe7q002@sneakemail.com> writes:
> On 3/26/07, Hannes Dorbath light-at-theendofthetunnel.de |postgresql|
>>> However, I am primarily concerned with safety/recoverability (on sudden power loss);
>>
>> Well then.. forget about NFS :)
> Could you offer any explanation as to why?
Basically, the problem with NFS is that it adds new failure modes that
are not present for local storage. Sure, if all works according to
spec then it's fine, but you're concerned about things going wrong no?
One of the nastier problems that we've seen involved an NFS-mounted DB
where the mount didn't come up till after the postmaster started.
(Searching in the PG archives should uncover that horror story among
others.) You can prevent some problems by being sure to mount the NFS
server hard not soft, but that's far from being a panacea.
AFAIK these issues are not PG-specific in the slightest, but are a
hazard for any DBMS run atop NFS.
regards, tom lane
On Mar 26, 2007, at 5:19 PM, Yang wrote: > On 3/26/07, A.M. agentm-at-themactionfaction.com |postgresql| > <...> wrote: >> >> On Mar 26, 2007, at 19:29 , Yang wrote: >> >> > >> > The environments involve two small devices - one with a flash disk >> > (the NFS server), and a slave which network-boots off that. Hence >> > these suggestions don't address the problem. (Would all the >> > alternative protocols listed at the top be able to coexist with the >> > described environment? Both devices must be able to boot into >> Linux.) >> >> Since you're booting from the NFS server, it would make more sense to >> have your boot process start a postgresql instance from a copy of the >> data directory instead of over NFS, no? Certainly, that way, you can >> have multiple instances booted and running. Do you need to sync back >> to the NFS server? > > The second device has no non-volatile storage. (Sorry I should've > explicitly stated this.) I might have missed something in the thread, but is there any reason why you can't run the database on the first device, using the local storage, and then connect to it from the second? Cheers, Steve