On 07/09/2010 13:02, Bert Huijben wrote:
-----Original Message-----
From: Matthew Bentham [mailto:mj...@artvps.com]
Sent: dinsdag 7 september 2010 13:48
To: Bert Huijben
Cc: 'Justin Erenkrantz'; 'Greg Stein'; 'Johan Corveleyn'; 'Subversion
Development'
Subject: Re: Worried about single-db performance
I didn't realise this, you are of course right that that would make it
unacceptable. I don't really understand why it would break
TortoiseSVN,
does it take write access and then not release it somehow?
SQLite needs a shared *read* lock to *read*. See
http://www.sqlite.org/atomiccommit.html.
(Invoking 'svn status' never obtains a write lock; see that document)
SQLite only upgrades that read lock to a write (or actually reserved) lock
when you perform a db operation that has to change the database. Further on
(E.g. too many changes, but look at the documentation for more reasons) this
is upgraded to an exclusive lock that blocks all readers and writers out of
the db, but it tries to keep this time as short as possible.
Your original suggestion is just to make any *reader* block any other
*reader*. Which breaks the subversion world. (Just running svn update in 1.6
has about 5 simultaneous independent readers in some phases of update. Most
GUI subversion clients I know use multiple client instances at the same
time, so they would all have to be rewritten if we obtain an exclusive lock
for reading).
Sorry, I didn't mean we should take exclusive locks for every
transaction, just that we should use "PRAGMA locking_mode=EXCLUSIVE".
According to the documentation (http://www.sqlite.org/pragma.html) that
makes transactions obtain a shared lock for reading which is upgraded to
an exclusive lock for writing, and not released until the database
connection is closed. I've tried it a couple of times in svn.exe and it
always improves performance (over locking_mode=NORMAL) and hasn't caused
me problems. Admittedly I haven't tried within the last couple of weeks
and I'm afraid I don't have time right now.
Am I misreading the documentation? It says "The first time the database
is read in EXCLUSIVE mode, a shared lock is obtained and held. The first
time the database is written, an exclusive lock is obtained and held".
Matthew
