bin/144446: [patch] db(3) fails with large block sizes
peterjeremy at acm.org
Wed Mar 3 11:30:02 UTC 2010
>Synopsis: [patch] db(3) fails with large block sizes
>Arrival-Date: Wed Mar 03 11:30:01 UTC 2010
>Originator: Peter Jeremy
>Release: FreeBSD 8.0-STABLE amd64
System: FreeBSD server.vk2pj.dyndns.org 8.0-STABLE FreeBSD 8.0-STABLE #1: Wed Jan 27 06:55:10 EST 2010 root at server.vk2pj.dyndns.org:/var/obj/usr/src/sys/server amd64
Whilst trying to port db(3) to a Solaris system, I have identified
two issues with the existing hash(3) code.
Firstly, when creating a new hash database, the bucket size
defaults to the st_blksize of the file (hash/hash.c::init_hash()).
There is no sanity checking to ensure that st_blksize is within
valid limits (hash/hash.h defined MAX_BSIZE as 65536).
In FreeBSD, st_blksize is currently hardwired to PAGE_SIZE in
kern/vfs_vnops.c::vn_stat() so this is purely a theoretical
issue on FreeBSD. Solaris exposes the blocksize from the
underlying filesystem - and in the case of ZFS, this is 128KB,
which exceeds MAX_BSIZE. In my case, the symptoms were that
when sequentially reading the database (via DB->seq()), the
returned keys were padded with 64KB of NULs.
Secondly, when the bucket size is set to 64KB (MAX_BSIZE),
non-trivial databases crash reporting:
"HASH: Out of overflow pages. Increase page size"
It's not clear what triggers this.
Thirdly, whilst writing this PR, I've noticed that hash(3)
states that the default hash table bucket size is 256 bytes.
The actual default (as per DEF_BUCKET_SIZE in hash/hash.h)
is 4096 bytes.
First problem can't be reproduced on FreeBSD but occurs on
at least Solaris and OpenSolaris with ZFS.
The second problem can be reproduced by extending the db
test tool (lib/libc/db/test/run.test, test20) to include
a bucket size of 65536. Based on the progression of
fill factor's in test20, the logical fill factors are
2735 3647 5471, however the test fails with each of these
values as well as 8, 341 and 10001. All other bucket
sizes appear to work successfully - which suggests there
is a problem with this particular bucket size.
First patch adds check for excessive st_blksize. Note that this
assumes that st_blksize is a power of 2. It might be reasonable
to verify that st_blksize isn't too small but I'm not sure what
the lower limit is.
RCS file: /usr/ncvs/src/lib/libc/db/hash/hash.c,v
retrieving revision 184.108.40.206
diff -u -r220.127.116.11 hash.c
--- hash/hash.c 28 Aug 2009 19:48:06 -0000 18.104.22.168
+++ hash/hash.c 3 Mar 2010 09:39:45 -0000
@@ -293,6 +293,8 @@
if (stat(file, &statbuf))
hashp->BSIZE = statbuf.st_blksize;
+ if (hashp->BSIZE > MAX_BSIZE)
+ hashp->BSIZE = MAX_BSIZE;
hashp->BSHIFT = __log2(hashp->BSIZE);
This is simply a work-around for the second problem. I'm not
certain what the actual problem is bu this avoids it.
RCS file: /usr/ncvs/src/lib/libc/db/hash/hash.h,v
retrieving revision 22.214.171.124
diff -u -r126.96.36.199 hash.h
--- hash/hash.h 3 Aug 2009 08:13:06 -0000 188.8.131.52
+++ hash/hash.h 3 Mar 2010 09:39:45 -0000
@@ -118,7 +118,7 @@
-#define MAX_BSIZE 65536 /* 2^16 */
+#define MAX_BSIZE 32768 /* 2^15 but should be 65536 */
#define MIN_BUFFERS 6
#define MINHDRSIZE 512
#define DEF_BUFSIZE 65536 /* 64 K */
The following updates the man page to contain the current
default bucket size as per DEF_BUCKET_SIZE in hash/hash.h
RCS file: /usr/ncvs/src/lib/libc/db/man/hash.3,v
retrieving revision 184.108.40.206
diff -u -r220.127.116.11 hash.3
--- man/hash.3 3 Aug 2009 08:13:06 -0000 18.104.22.168
+++ man/hash.3 3 Mar 2010 09:50:52 -0000
@@ -78,7 +78,7 @@
-table bucket size, and is, by default, 256 bytes.
+table bucket size, and is, by default, 4096 bytes.
It may be preferable to increase the page size for disk-resident tables
and tables with large data items.
.It Va ffactor
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