Chris Mason, maybe you might want to comment on what he is doing with threads and how it interacts with V3 journal commits? (I bet you are on vacation though?)
Hans
Chris Haynes wrote:
Hans,
Thanks for asking!
I'm about to deploy an on-line transaction-based service.. All
service-specific software is in 1000+ Java classes. The OS is SuSE
7.3, and Java version 1.4
The heavist uses of the file store are:
During development:
+ javadoc
During service deployment / update:
+ rsync
+ When using a tripwire-like Java program which checks the SHA digest
of all deployable files against zipped JARs
During production operation:
+ atomic, synchronized writes to multiple files (typically 3 - 4 files
in different directories, the first is the creation of a new 4 kb
file, others are usually updates of existing files - growing typically
1kb per update). This files system is mounted on a RAID-1 pair.
+ rsync
+ Successive reading of all files in a directory sub-tree (up to 10M
files)
in filestore-defined order (i.e. the program makes no demands or
assumptions about the order - it uses the order supplied by Java's
File.files()).
The greatest performance concern I have is with the file writes. As
these are atomic transactions, I use a separate thread for each file's
write (to give the kernel's escalator a chance to work), and require
that the write operations be individually hardware-synchronized using
Java's FileDescriptor.sync() method. I then use a counter to detect
when all threads have reported that their files have been written -
this indicating successful commitment of the transaction.
I handle read-and write-locking in the application.
Usually, there are no lock conflicts, so there can be many concurrent
transaction commitments. I use a thread pool of 50 threads to handle
the individual file writes (the 50 being a guess at the likely point
of diminishing returns).
My expectation/hope is that, so long as there are enough threads
available in this pool, all transactions will be completed within one
disk rotation period(regardless of the number of concurrent
transactions or number of files per transaction and the fact that I'm
using software RAID-1). I've not yet been able to validate this
(theoretically or practically).
I would *really* like to be able to group all the file writes for a
transaction into a single logical API call and have the kernel/file
system report successful completion of all data and metadata aspects
of the transaction using a single application thread.
HTH
Chris Haynes
