On 2018-02-28 19:08, Max Reitz wrote: > On 2018-02-27 17:17, Stefan Hajnoczi wrote: >> On Mon, Feb 26, 2018 at 06:03:13PM +0100, Max Reitz wrote: >>> There are filesystems (among which is tmpfs) that have a hard time >>> reporting allocation status. That is definitely a bug in them. >>> >>> However, there is no good reason why qemu-img convert should query the >>> allocation status in the first place. It does zero detection by itself >>> anyway, so we can detect unallocated areas ourselves. >>> >>> Furthermore, if a filesystem driver has any sense, reading unallocated >>> data should take just as much time as lseek(SEEK_DATA) + memset(). So >>> the only overhead we introduce by dropping the manual lseek() call is a >>> memset() in the driver and a buffer_is_zero() in qemu-img, both of which >>> should be relatively quick. >> >> This makes sense. Which file systems did you test this patch on? > > On tmpfs and xfs, so far. > >> XFS, ext4, and tmpfs would be a good minimal test set to prove the >> patch. Perhaps with two input files: >> 1. A file that is mostly filled with data. >> 2. A file that is only sparsely populated with data. > > And probably with vmdk, which (by default) forbids querying any areas > larger than 64 kB. > >> The time taken should be comparable with the time before this patch. > > Yep, I'll do some benchmarks.
And the results are in. I've created 2 GB images on various filesystems in various formats, then I've either written 64 kB every 32 MB to them ("sparse"), or left out 64 kB every 32 MB ("full"). Then I've converted them to null-co:// and took the (real) time through "time". (Script is attached.) I've attached the raw results before and after this patch. Usually, I did six runs for each case and dropped the most extreme outlier -- except for full vmdk images, where I've only done one run for each case because creating these images can take a very long time. Here are the differences from before to after: sparse raw on tmpfs: + 19 % (436 ms to 520 ms) sparse qcow2 on tmpfs: - 31 % (435 ms to 301 ms) sparse vmdk on tmpfs: + 37 % (214 ms to 294 ms) sparse raw on xfs: + 69 % (452 ms to 762 ms) sparse qcow2 on xfs: - 34 % (462 ms to 304 ms) sparse vmdk on xfs: + 42 % (210 ms to 298 ms) sparse raw on ext4: +360 % (144 ms to 655 ms) sparse qcow2 on ext4: +120 % (147 ms to 330 ms) sparse vmdk on ext4: + 16 % (253 ms to 293 ms) full raw on tmpfs: - 9 % (437 ms to 398 ms) full qcow2 on tmpfs: - 75 % (1.63 s to 403 ms) full vmdk on tmpfs: -100 % (10 min to 767 ms) full raw on xfs: - 1 % (407 ms to 404 ms, insignificant) full qcow2 on xfs: - 1 % (410 ms to 404 ms, insignificant) full vmdk on xfs: - 33 % (1.05 s to 695 ms) full raw on ext4: - 2 % (308 ms to 301 ms, insignificant) full qcow2 on ext4: + 2 % (307 ms to 312 ms, insignificant) full vmdk on ext4: - 74 % (3.53 s to 839 ms) So... It's more extreme than I had hoped, that's for sure. What I conclude from this is: (1) This patch is generally good for nearly fully allocated images. In the worst case (on well-behaving filesystems with well-optimized image formats) it changes nothing. In the best case, conversion time is reduced drastically. (2) For sparse raw images, this is absolutely devastating. Reading them now takes more than (ext4) or nearly (xfs) twice as much time as reading a fully allocated image. So much for "if a filesystem driver has any sense". (2a) It might be worth noting that on xfs, reading the sparse file took longer even before this patch... (3) qcow2 is different: It benefits from this patch on tmpfs and xfs (note that reading a sparse qcow2 file took longer than reading a full qcow2 file before this patch!), but it gets pretty much destroyed on ext4, too. (4) As for sparse vmdk images... Reading them takes longer, but it's still fasster than reading full vmdk images, so that's not horrible. So there we are. I was wrong about filesystem drivers having any sense, so this patch can indeed have a hugely negative impact. I would argue that conversion time for full images is more important, because that's probably the main use case; but the thing is that here this patch only helps for tmpfs and vmdk. We don't care too much about vmdk, and the fact that tmpfs takes so long simply is a bug. I guess the xfs/tmpfs results would still be in a range where they are barely acceptable (because it's mainly a qcow2 vs. raw tradeoff), but the ext4 horrors probably make this patch a no-go in its current form. In any case it's interesting to see that even the current qemu-img convert takes longer to read sparsely allocated qcow2/raw files from xfs than fully allocated images... So I guess I'll re-send this patch where the change is done only for -S 0. Max
test.sh
Description: application/shellscript
Testing: raw on tmpfs (sparse): 0.436 ±0.012 real 0.426 0.444 0.447 0.442 (0.500) 0.421 Testing: qcow2 on tmpfs (sparse): 0.435 ±0.018 real 0.421 0.459 0.448 0.430 (0.776) 0.415 Testing: vmdk on tmpfs (sparse): 0.214 ±0.004 real 0.216 0.218 0.210 0.215 (0.222) 0.209 Testing: raw on xfs (sparse): 0.452 ±0.028 real 0.430 0.468 0.493 0.442 (0.640) 0.428 Testing: qcow2 on xfs (sparse): 0.462 ±0.016 real 0.445 0.475 0.478 0.445 0.466 (0.437) Testing: vmdk on xfs (sparse): 0.210 ±0.005 real 0.214 0.213 (0.222) 0.203 0.207 0.213 Testing: raw on ext4 (sparse): 0.144 ±0.023 real 0.119 0.181 0.142 0.143 0.137 (0.296) Testing: qcow2 on ext4 (sparse): 0.147 ±0.016 real 0.139 0.138 0.142 0.175 0.140 (0.301) Testing: vmdk on ext4 (sparse): 0.253 ±0.004 real 0.247 (0.273) 0.257 0.252 0.254 0.254 Testing: raw on tmpfs (full): 0.437 ±0.012 real (0.477) 0.436 0.434 0.432 0.457 0.424 Testing: qcow2 on tmpfs (full): 1.630 ±0.021 real 1.618 1.631 (1.684) 1.625 1.665 1.612 Testing: vmdk on tmpfs (full): 612.379 (n=1) real 612.379 Testing: raw on xfs (full): 0.407 ±0.011 real 0.400 0.413 0.402 (0.495) 0.424 0.397 Testing: qcow2 on xfs (full): 0.410 ±0.009 real 0.403 0.406 0.413 (0.485) 0.423 0.403 Testing: vmdk on xfs (full): 1.048 (n=1) real 1.048 Testing: raw on ext4 (full): 0.308 ±0.007 real 0.299 (0.419) 0.315 0.311 0.301 0.314 Testing: qcow2 on ext4 (full): 0.307 ±0.003 real 0.307 0.306 (0.490) 0.308 0.303 0.312 Testing: vmdk on ext4 (full): 3.528 (n=1) real 3.528
Testing: raw on tmpfs (sparse): 0.520 ±0.022 real 0.496 0.535 0.536 0.538 (0.569) 0.495 Testing: qcow2 on tmpfs (sparse): 0.301 ±0.008 real 0.298 0.297 (0.338) 0.292 0.307 0.312 Testing: vmdk on tmpfs (sparse): 0.294 ±0.002 real 0.295 0.294 0.292 (0.287) 0.293 0.297 Testing: raw on xfs (sparse): 0.762 ±0.026 real (0.713) 0.770 0.769 0.759 0.790 0.720 Testing: qcow2 on xfs (sparse): 0.304 ±0.003 real 0.309 (0.312) 0.301 0.302 0.306 0.304 Testing: vmdk on xfs (sparse): 0.298 ±0.009 real 0.294 0.312 0.293 0.301 0.290 (0.345) Testing: raw on ext4 (sparse): 0.655 ±0.011 real 0.664 (0.624) 0.663 0.639 0.648 0.659 Testing: qcow2 on ext4 (sparse): 0.330 ±0.035 real 0.293 0.333 (0.391) 0.374 0.353 0.297 Testing: vmdk on ext4 (sparse): 0.293 ±0.004 real 0.290 0.289 0.293 0.297 (0.285) 0.296 Testing: raw on tmpfs (full): 0.398 ±0.004 real 0.397 (0.406) 0.396 0.397 0.394 0.404 Testing: qcow2 on tmpfs (full): 0.403 ±0.004 real 0.403 0.408 0.406 0.399 (0.538) 0.400 Testing: vmdk on tmpfs (full): 0.767 (n=1) real 0.767 Testing: raw on xfs (full): 0.404 ±0.004 real 0.411 0.402 0.401 0.402 0.402 (0.421) Testing: qcow2 on xfs (full): 0.404 ±0.004 real 0.402 (0.415) 0.410 0.401 0.401 0.404 Testing: vmdk on xfs (full): 0.695 (n=1) real 0.695 Testing: raw on ext4 (full): 0.301 ±0.004 real 0.304 0.303 0.305 0.299 0.296 (0.322) Testing: qcow2 on ext4 (full): 0.312 ±0.008 real (0.325) 0.310 0.319 0.304 0.305 0.321 Testing: vmdk on ext4 (full): 0.839 (n=1) real 0.839
signature.asc
Description: OpenPGP digital signature