On Sun, Aug 24, 2014 at 06:47:21PM -0400, Theodore Ts'o wrote: > On Fri, Aug 08, 2014 at 09:26:30PM -0700, Darrick J. Wong wrote: > > The block_validity mount option spot-checks block allocations against > > a bitmap of known group metadata blocks. This helps us to prevent > > self-inflicted catastrophic failures such as trying to "share" > > critical metadata (think bitmaps) with file data, which usually > > results in filesystem destruction. > > > > In order to test the overhead of the mount option, I re-used the speed > > tests in the metadata checksum testing script. In short, the program > > creates what looks like 15 copies of a kernel source tree, except that > > it uses fallocate to strip out the overhead of writing the file data > > so that we can focus on metadata overhead. On a 64G RAM disk, the > > overhead was generally about 0.9% and at most 1.6%. On a 160G USB > > disk, the overhead was about 0.8% and peaked at 1.2%. > > I was doing a spot check of the additional memory impact of > block_validity mount option, and it's for a 20T file system, assuming > the basic flex_bg size of 16 block groups, it's a bit over 400k of > kernel memory. That's not a *huge* amount of memory, but it could > potentially be noticeable on a bookshelf NAS server. > > However, I could imagine that for a system with say, two dozen 10T > drives (which aren't that far off in the future) in a tray, that's > around 4 megabytes of memory, which starts being non-trivial. > > That being said, I suspect for most users, it's not that big of a deal > --- so maybe this is something we should just simply enable by default > in the kernel, let those folks who want to disable specify a > noblock_validity mount option. Should there be a noblock_validity default mount option? I suppose I can simply send in a one-liner making b_v the kernel default and see if anyone screams.... > The other thing to consider is that for big raid arrays, maybe we > should use a larger flex_bg size. The main reason for keeping the > size small is to minimize the seek time between the inode table and a > block in the flex_bg. But for raid devices, we could probably afford > to increase flex_bg size, which would decrease the numer of system > zones that the block validity code would need to track. One could make the default flexbg size = 16 * stride_width / stripe_width as a start. --D > > - Ted > -- > To unsubscribe from this list: send the line "unsubscribe linux-ext4" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
