Date: Sat, 22 Jan 2011 10:39:13 +0000 From: Karl Pielorz <kpielorz_lst@tdx.co.uk> To: freebsd-fs@freebsd.org Subject: Write cache, is write cache, is write cache? Message-ID: <1ABA88EDF84B6472579216FE@Octa64>
next in thread | raw e-mail | index | archive | help
Hi, I've a small HP server I've been using recently (an NL36). I've got ZFS setup on it, and it runs quite nicely. I was using the server for zeroing some drives the other day - and noticed that a: dd if=/dev/zero of=/dev/ada0 bs=2m Gives around 12Mbyte/sec throughput when that's all that's running on the machine. Looking in the BIOS is a "Enabled drive write cache" option - which was set to 'No'. Changing it to 'Yes' - I now get around 90-120Mbyte/sec doing the same thing. Knowing all the issues with IDE drives and write caches - is there any way of telling if this would be safe to enable with ZFS? (i.e. if the option is likely to be making the drive completely ignore flush requests?) - or if it's still honouring the various 'write through' options if set on data to be written? I'm presuming DD won't by default be writing the data with the 'flush' bit set - as it probably doesn't know about it. Is there anyway of testing this? (say using some tool to write the data using either lots of 'cache flush' or 'write through' stuff) - and seeing if the performance drops back to nearer the 12Mbyte/sec? I've not enabled the option with the ZFS drives in the machine - I suppose I could test it. Write performance on the unit isn't that bad [it's not stunning] - though with 4 drives in a mirrored set - it probably helps hide some of the impact this option might have. -Kp
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?1ABA88EDF84B6472579216FE>