T
yarono wrote:
My OS is suse10.
my JVM is IBM's version 1.4.2 as detailed in the following link:
http://www.novell.com/products/linuxpackages/server10/i386/java-1_4_2-ibm-jdbc.html
The app is indeed single-threaded, so group commit is not the issue.
The synchronous write was measured in c (not in java).
Is there a way to control or configure the synchronization of writes of the
JVM?
not in the JVM, but what we have seen is that there are OS/disk drive
parameters that can be set which disable actually executing a sync when
a program uses certain interfaces to do the sync. You don't say what
specific interface you used in c to do the sync, so I can't comment on that.
In windows what we found is that there is the ability on a per disk
drive to set a flag called "write cache enabled". Basically it says
when write hit device driver return immediately even if program has
requested
no return until write has synced to platter. What is more confusing
is that in java there are 2 ways to do a sync:
1) sync the whole file
2) sync every time you do write.
#1 is not affected by "write cache enabled"
#2 is affected by "write cache enabled"
In jvm's of 1.4.2 and higher Derby uses method 2 to sync the log on
commit.
I know there is a similar affect on c programs on various OS's but don't
remember the details, basically we observed the same no syncing on some
other db products when write cache enabled was set.
My guess is that suse10 has some similar setting, maybe on file system
rather than device? Here is a random link I came up with about
write cache's and linux, I have no idea if it is valid but a google
search on "disable write cache linux" gives lots of hits:
http://www.jasonbrome.com/blog/archives/2004/04/03/writecache_enabled.html
Mike Matrigali wrote:
is your app single threaded, if so group commit is not the issue.
What is your OS? What is your JVM? Derby may use different syncing
algorithms depending on JVM version.
How did you measure synchronous write, ie. did you
write a java program and execute against the same JVM as derby is
running in?
The disk that that contains the log directory is the one of interest.
Each transaction is made up of a number of log records. From your
description each transaction will have the following:
begin log record
insert log record for row into base table
insert log record for row into primary key index
commit log record
yarono wrote:
Hello,
I'm working on a simple db. Each record is composed of 3 long values. The
first two are the primary key.
I have to measure the performance of the insertions. Each insertion is
wrapped in a transaction, which is commited having only one insertion in
it.
I've measured both berkeley db performance and postgres and got about
110-115 insertions per second.
Now in derby db (both in embeded mode and server mode) I get better
performance: about 250-300 insertions per second. This obviously results
from some kind of a group commit, although I get these results both when
auto-commiting or manual-commiting after each insertion.
I've performed a simple test of synchronious writing 24 bytes (3 * 8
bytes)
to the disk. It measure 117 writes per second, and I believe this is the
upper bound of any db performance.
So, I don't understand why I get such good performance, although I commit
after each insertion.
I examined the .dat files in both /log and /seg0 folders. None of them
increase in 24 bytes segments, but rathar bigger segments.
So, my questions are:
1. Which log file in /log or /seg0 should I examine to analyze the numebr
of
bytes written each write to disk?
2. How do I disable the group commit or whatever attribute that causes
this
communal write? how do I make each transaction be written on its own to
the
disk?
Thanks in advance,
Yaron
