Hi, I have a server that has basically two tasks. 1) Receiving lots of data from the network and storing it on disk. 2) An App that makes relatively small use of the disk and responds to requests from the network. The problem I have is that sometimes (1) is filling up all the "Dirty" pages, triggering a blocking flushing of the dirty buffer to the disk. This essentially freezes (1) and (2) until the flushing is complete. On occasions, this can take more than 60 seconds. 60 seconds is far too long from (2) point of view, because it needs to respond to user requests quickly, i.e less than 1 second. Is there any mechanism that could result in (1) being informed about the problem, (1) could then back off writing data to disk, and then at the same time, asked the sending system over the network to also back off. On TCP/IP networks, this is reported back as "congestion" on the network, the this results in throttling of the sending application on a per TCP session basis. In the above case, we are essentially seeing "congestion" to a particular storage disk, but the application does not get any feedback about this. I guess the perfect solution would be Quality-of-Service for disk writes, much like we have for network traffic. So, is there a feature available that can help me here, or will I have to look at modifying the Linux kernel in order to add support for "congestion notification from disk writes" ? In my view that "dirty_ratio" causing the whole system to appear to freeze due to disk blocking is too blunt an instrument. Also, even detecting if the 60 second freezes are a result of the "dirty_ratio" being hit is difficult to do. It would be useful if there existed a counter that would count the amount of times the system resorted to "blocking" writes, as opposed to the non-problematic background writes. In my view, whenever the "blocking" writes was initiated, the application should be informed about it. Another alternative could be that the dirty pages are associated with the application process and file descriptor and a dirty_ratio set per file descriptor. Then, when a dirty_ratio is hit on the file descriptor, only the application that holds that fd is frozen. Maybe have multi-level limits. I.e. Warn App at limit A, freeze app at limit B. Kind Regards James
