>>>>> "Zdenek" == Zdenek Kabelac <zkabelac@xxxxxxxxxx> writes: Zdenek> Dne 09. 09. 20 v 20:47 John Stoffel napsal(a): >>>>>>> "Gionatan" == Gionatan Danti <g.danti@xxxxxxxxxx> writes: >> Gionatan> Il 2020-09-09 17:01 Roy Sigurd Karlsbakk ha scritto: >>>> First, filelevel is usually useless. Say you have 50 VMs with Windows >>>> server something. A lot of them are bound to have a ton of equal >>>> storage in the same areas, but the file size and content will vary >>>> over time. With blocklevel tiering, that could work better. >> Gionatan> It really depends on the use case. I applied it to a Gionatan> fileserver, so working at file level was the right Gionatan> choice. For VMs (or big files) it is useless, I agree. >> >> This assumes you're tiering whole files, not at the per-block level >> though, right? >> >>>> This is all known. >> Gionatan> But the only reason to want tiering vs cache is the Gionatan> additional space the former provides. If this additional Gionatan> space is so small (compared to the combined, total volume Gionatan> space), tiering's advantage shrinks to (almost) nothing. >> >> Do you have numbers? I'm using DM_CACHE on my home NAS server box, >> and it *does* seem to help, but only in certain cases. I've got a >> 750gb home directory LV with an 80gb lv_cache writethrough cache >> setup. So it's not great on write heavy loads, but it's good in read >> heavy ones, such as kernel compiles where it does make a difference. >> >> So it's not only the caching being per-file or per-block, but how the >> actual cache is done? writeback is faster, but less reliable if you >> crash. Writethrough is slower, but much more reliable. Zdenek> dm-cache (--type cache) is hotspot cache (most used areas of device) I assume this is what I'm using on Debian Buster (10.5) right now? I use the crufty tool 'lvcache' to look at and manage my cache devices. Zdenek> dm-writecache (--type writecache) is great with Zdenek> write-extensive load (somewhat extends your page cache on your Zdenek> NMVe/SSD/persistent-memory) I don't think I'm using this at all: sudo lvcache status data/home +-----------------------+------------------+ | Field | Value | +-----------------------+------------------+ | cached | True | | size | 806380109824 | | cache_lv | home_cache | | cache_lv_size | 85899345920 | | metadata_lv | home_cache_cmeta | | metadata_lv_size | 83886080 | | cache_block_size | 192 | | cache_utilization | 873786/873813 | | cache_utilization_pct | 99.996910094 | | demotions | 43213 | | dirty | 0 | | end | 1574961152 | | features | 1 | | md_block_size | 8 | | md_utilization | 2604/20480 | | md_utilization_pct | 12.71484375 | | promotions | 43208 | | read_hits | 138697828 | | read_misses | 7874434 | | segment_type | cache | | start | 0 | | write_hits | 777455171 | | write_misses | 9841866 | +-----------------------+------------------+ Zdenek> We were thinking about layering cached above each other - but so far there Zdenek> was no big demand and also the complexity of solving problem is rising greatly Zdenek> - aka there is no problem to let users to stack cache on top of another cache Zdenek> on top of 3rd. cache - but what should have when it starts failing... Zdenek> AFAIK there is no one yet writing driver for combining i.e. SSD + HDD Zdenek> into a single drive which would be relocating blocks (so you get total size as Zdenek> aproximate sum of both devices) - but there is dm-zoned which solves somewhat Zdenek> similar problem - but I've no experience with that... Zdenek> Zdenek _______________________________________________ linux-lvm mailing list linux-lvm@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
