Austin S. Hemmelgarn posted on Wed, 28 Feb 2018 14:24:40 -0500 as excerpted:
>> I believe this effect is what Austin was referencing when he suggested >> the defrag, tho defrag won't necessarily /entirely/ clear it up. One >> way to be /sure/ it's cleared up would be to rewrite the entire file, >> deleting the original, either by copying it to a different filesystem >> and back (with the off-filesystem copy guaranteeing that it can't use >> reflinks to the existing extents), or by using cp's --reflink=never >> option. >> (FWIW, I prefer the former, just to be sure, using temporary copies to >> a suitably sized tmpfs for speed where possible, tho obviously if the >> file is larger than your memory size that's not possible.) > Correct, this is why I recommended trying a defrag. I've actually never > seen things so bad that a simple defrag didn't fix them however (though > I have seen a few cases where the target extent size had to be set > higher than the default of 20MB). Good to know. I knew larger target extent sizes could help, but between not being sure they'd entirely fix it and not wanting to get too far down into the detail when the copy-off-the-filesystem-and-back option is /sure/ to fix the problem, I decided to handwave that part of it. =:^) > Also, as counter-intuitive as it > might sound, autodefrag really doesn't help much with this, and can > actually make things worse. I hadn't actually seen that here, but suspect I might, now, as previous autodefrag behavior on my system tended to rewrite the entire file[1], thereby effectively giving me the benefit of the copy-away-and-back technique without actually bothering, while that "bug" has now been fixed. I sort of wish the old behavior remained an option, maybe radicalautodefrag or something, and must confess to being a bit concerned over the eventual impact here now that autodefrag does /not/ rewrite the entire file any more, but oh, well... Chances are it's not going to be /that/ big a deal since I /am/ on fast ssd, and if it becomes one, I guess I can just setup say firefox-profile-defrag.timer jobs or whatever, as necessary. --- [1] I forgot whether it was ssd behavior, or compression, or what, but something I'm using here apparently forced autodefrag to rewrite the entire file, and a recent "bugfix" changed that so it's more in line with the normal autodefrag behavior. I rather preferred the old behavior, especially since I'm on fast ssd and all my large files tend to be write- once no-rewrite anyway, but I understand the performance implications on large active-rewrite files such as gig-plus database and VM-image files, so... -- Duncan - List replies preferred. No HTML msgs. "Every nonfree program has a lord, a master -- and if you use the program, he is your master." Richard Stallman -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
