On Sun, Jan 24, 2010 at 10:23:46PM -0500, nixlists wrote: > On Sun, Jan 24, 2010 at 9:18 PM, Marco Peereboom <sl...@peereboom.us> 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. > > 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. > 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. > >> > Hope you now know that virtually all PATA & SATA have WB cache enabled. > >> > >> Of course I know, as was stated in the previous message, but of > >> course, as most people, I disable it. > >> Don't twist what I said. If you read the previous email again, you'll > >> see that I say "no write-back cache.". > > > > And you can repeat this all day long but you simply can not make these > > assumptions. Yes in theory this would work but that damn reality is so > > freaking unpredictable. Someone write a patch for that. > > Let's all roll-over and die - we might die any second anyway because > nothing is guaranteed, so why stay alive? Are thousands of people > running mail servers losing messages in crashes all the time, and are > unaware of it? 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. 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. > > > those "can you write me some code that works around those annoying > > signaling issues?" person. > > Nope. > > Thanks!
