On Sun, Jan 24, 2010 at 11:14 PM, Marco Peereboom <[email protected]> wrote: > On Sun, Jan 24, 2010 at 10:23:46PM -0500, nixlists wrote: >> On Sun, Jan 24, 2010 at 9:18 PM, Marco Peereboom <[email protected]> wrote: >> >> I specifically wrote above "When configured as documented." No admin >> >> will run a mail server with write-back cache enabled on either >> >> controller or drives (well, maybe with a battery back-up, but I'll say >> >> again that batteries fail too). You seem to be taking what I wrote out >> >> of context, or you are assuming that I am a moron who doesn't know the >> >> basics and run mail servers with write-back cache on controllers and >> >> drives. >> > >> > No one disables WB cache for 2 reasons: >> >> Are you speaking for everybody? This is simply not true. >> >> > 1. They don't know how >> >> Unless I am missing something, this is not true... I disable it, It's >> right in my RAID controller's config. > > Congratulations you disabled the write cache for the raid controller. > Disks are often not available through the controllers config and you > need special tools to accomplish that. Some vendors do humor you and > provide it. > >> Or, are you trying to say that the RAID controller doesn't honor what >> I am telling it to do? A benchmark seems to tell me otherwise... Now, >> forget RAID, what about simple SATA controllers that are built into >> the motherboard? Simple SATA add-on cards (non-softRAID, non-RAID)? Do >> they even have cache? > > It is still RAID and you lost control over your IO. Do some math and > figure out how many backend IOs a 1 block sized frontend IO takes. > Repeat for RAID 5 & 6; oh and show the world how clever you are and try > it to for a RAID 6 set that misses the ECC block and/or the parity > block.
RAID 1 only in my case. >> > 2. They are disappointed with the floppy disk like performance. >> > Bonus: drive vendors tell you not to do it. >> >> Performance and vendors are different issues. Let's stay on the topic >> of rename() guarantee as in the man page during a crash or powerfail, > > I am on topic. Every single HDD mfg tells you to enable WB cache on > SATA drives. Doesn't mean people don't disable write-back cache for obvious reasons. >> provided that the controller is configured not to write-back cache, >> the drives are configured not to write-back cache, the FS is mounted >> 'sync'. No softupdates. Let's not divert this to something tangential >> and unrelated. I'll take reliability over performance. > > You play with RAID you lose. You play with anything other than a > straight from OS memory to platter and you lose. Which is about > everything these days. FIne then, according to you it's every single RAID controller in the world that cannot be trusted. Now the simplest case: a SATA controller as found on any recent motherboard, or a SATA add-on card, and a disk with write-back cache turned off. What are the problems there? > No. People understand the risks and mitigate them as much as possible > by using technologies that make sense for their budget and requirements. > They don't go on mailing lists asserting that generic software can do > ungeneric things to an arbitrary piece of hardware. > > Another fun read is the HDD mfgs small print. Try finding in there that > they'll actually guarantee anything on that disk. Good luck. > >> >> >> Please, point me to hardware that, when met all the above conditions, >> >> is still unreliable for rename(). It would benefit thousands of people >> >> running mail servers. >> > >> > All RAID controllers. And I mean every single last one of them. >> > Including external RAID cards too. You have exactly zero control as to >> > what they do. Write/Back/Through etc they are going to sit on your data >> > regardless of whatever the fruit you want. >> >> I am not sure what you are saying here. Are you saying people disable >> WB cache on controllers and disks (I know I do, and I know many others >> do), but it's still enabled? In other words, if I explicitly tell the >> controller and disks to disable write-back cache, and I can see it >> with benchmarks (write performance drops significantly,and the disk is >> much busier on writes), that they still do write-back caching? What >> about simple SATA? PATA? Granted I may not be aware of the nuances of >> controller and disk caching, but you I am sure do, and can can explain >> those. > > Well what I am saying is that you do not understand how RAID or other > "intelligent" IO machinery works. And I am telling you to stop making a > fool out of yourself repeating some assertions that are incorrect. But you still haven't answered the question asked. Again, given that write-back cache is disabled on both the controller and the disks - what problems are there? > NO the sky isn't falling and we all have mail. Pretty awesome we don't > have that many issues eh? Oh and keep a backup, you might need it.
