Re: fsync vs open_sync - Mailing list pgsql-performance

>> >     There is also the fact that NTFS is a very slow filesystem, and
>> > Linux is
>> > a lot better than Windows for everything disk, caching and IO related.
>> Try
>> > to copy some files in NTFS and in ReiserFS...
>>
>> I'm not so sure I would agree with such a blanket generalization.  I
>> find
>> NTFS to be very fast, my main complaint is fragmentation issues...I bet
>> NTFS is better than ext3 at most things (I do agree with you about the
>> cache, thoughO.
>
> Ok, you were right.  I made some tests and NTFS is just not very good in
> the general case.  I've seen some benchmarks for Reiser4 that are just
> amazing.

    As a matter of fact I was again amazed today.
    I was looking into a way to cache database queries for a website (not
yet) written in Python. The purpose was to cache long queries like those
used to render forum pages (which is the typical slow query, selecting
 from a big table where records are rather random and LIMIT is used to cut
the result in pages).
    I wanted to save a serialized (python pickled) representation of the data
to disk to avoid reissuing the query every time.
    In the end it took about 1 ms to load or save the data for a page with 40
posts... then I wondered, how much does it take just to read or write the
file ?

    ReiserFS 3.6, Athlon XP 2.5G+, 512Mb DDR400
    7200 RPM IDE Drive with 8MB Cache
    This would be considered a very underpowered server...

    22 KB files, 1000 of them :
    open(), read(), close() : 10.000 files/s
    open(), write(), close() : 4.000 files/s

    This is quite far from database FS activity, but it's still amazing,
although the disk doesn't even get used. Which is what I like in Linux.
You can write 10000 files in one second and the HDD is still idle... then
when it decides to flush it all goes to disk in one burst.

    I did make benchmarks some time ago and found that what sets Linux apart
 from Windows in terms of filesystems is :
    - very high performance filesystems like ReiserFS
    This is the obvious part ; although with a huuuuge amount of data in
small files accessed randomly, ReiserFS is faster but not 10x, maybe
something like 2x NTFS. I trust Reiser4 to offer better performance, but
not right now. Also ReiserFS lacks a defragmenter, and it gets slower
after 1-2 years (compared to 1-2 weeks with NTFS this is still not that
bad, but I'd like to defragment and I cant). Reiser4 will fix that
apparently with background defragger etc.

    - caching.
    Linux disk caching is amazing. When copying a large file to the same disk
on Windows, the drive head swaps a lot, like the OS can't decide between
reading and writing. Linux, on the other hand, reads and writes by large
chunks and loses a lot less time seekng. Even when reading two files at
the same time, Linux reads ahead in large chunks (very little performance
loss) whereas Windows seeks a lot. The read-ahead and write-back thus gets
it a lot faster than 2x NTFS for everyday tasks like copying files,
backing up, making archives, grepping, serving files, etc...
    My windows box was able to saturate a 100Mbps ethernet while serving one
large FTP file on the LAN (not that impressive, it's only 10 MB/s hey!).
However, when several simultaneous clients were trying to download
different files which were not in the disk cache, all hell broke loose :
lots of seeking, and bandwidth dropped to 30 Mbits/s. Not enough
read-ahead...
    The Linux box, serving FTP, with half the RAM (256 Mb), had no problem
pushing the 100 Mbits/s with something like 10 simultaneous connections.
The amusing part is that I could not use the Windows box to test it
because it would choke at such a "high" IO concurrency (writing 10
MBytes/s to several files at once, my god).
    Of course the files which had been downloaded to the Windows box were cut
in as many fragments as the number of disk seeks during the download...
several hundred fragments each... my god...

    What amazes me is that it must just be some parameter somewhere and the
Microsoft guys probably could have easily changed the read-ahead
thresholds and time between seeks when in a multitasking environment, but
they didn't. Why ?

    Thus people are forced to buy 10000RPM SCSI drives for their LAN servers
when an IDE raid, used with Linux, could push nearly a Gigabit...

    For database, this is different, as we're concerned about large files,
and fsync() times... but it seems reiserfs still wins over ext3 so...

    About NTFS vs EXT3 : ext3 dies if you put a lot of files in the same
directory. It's fast but still outperformed by reiser.

    I saw XFS fry eight 7 harddisk RAID bays. The computer was rebooted with
the Reset button a few times because a faulty SCSI cable in the eighth
RAID bay was making it hang. The 7 bays had no problem. When it went back
up, all the bays were in mayhem. XFSrepair just vomited over itself and we
got plenty of files with random data in them. Fortunately there was a
catalog of files with their checksums so at least we could know which
files were okay. Have you tried restoring that amount of data from a
backup ?

    Now maybe this was just bad luck and crap hardware, but I still won't
touch XFS. Amazing performance on large files though...

    I've had my computers shutdown violently by power failures and no
reiserfs problems so far. NTFS is very crash proof too. My windows machine
bluescreens twice a day and still no data loss ;)

    Upside : an junkyard UPS with dead batteries, powered with two brand new
12V car batteries, costs 70 euro and powers a computer for more than 5
hours...
    Downside :
    - it's ugly (I hide it under my desk)
    - you "borrow" a battery to start your friend's car, and just five
minutes later, the UPS wants to test itself, discovered it has no more
batteries, and switches everything off... argh.

    Good evening...