Date: Wed, 18 Oct 2023 15:37:59 +0000 From: "John F Carr" <jfc@mit.edu> To: void <void@f-m.fm> Cc: "freebsd-fs@freebsd.org" <freebsd-fs@freebsd.org> Subject: Re: free space considerations writing bhyve image to a zvol Message-ID: <1ACB40C4-6EA3-45AE-AC62-1D5ED71C22FF@mit.edu> In-Reply-To: <ZS_WZD-JlHEo_Em5@int21h> References: <ZS_WZD-JlHEo_Em5@int21h>
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> On Oct 18, 2023, at 08:58, void <void@f-m.fm> wrote: >=20 > A linux file-backed vm was created initially like this: >=20 > truncate -s 2T linuxvm.img >=20 > Within the vm, df -h reports 1.1Tb used in total. > Externally, on the freebsd host, the linuxvm.img is 2Tb. >=20 > If i write the vm to a 1.6Tb zvol with compression set (lz4), > would it be feasible to expect the vm to work? >=20 > I expect it might break the vm's filesystem internally, > now, thinking about it. > --=20 >=20 By "zvol" do you mean a volume created with zfs create -V? If so, compression does not affect how much data you can store before the volume becomes full. # zfs create -V 32M data/scratch # dd if=3D/dev/zero of=3D/dev/zvol/data/scratch bs=3D1m dd: /dev/zvol/data/scratch: end of device 33+0 records in 32+0 records out 33554432 bytes transferred in 0.021722 secs (1544723175 bytes/sec) By default zfs reserves enough space in the pool to allow the volume to be filled with uncompressed data. If you want a volume to hold 2 TB of logica= l data and you expect the compressed data to fit in 1.6 TB, set the refreservation property on the volume to the smaller size. See the discussion of the volsize parameter in zfsprops(7). https://man.freebsd.org/cgi/man.cgi?zfsprops # zfs set volsize=3D64M refreservation=3D32M data/scratch # dd if=3D/dev/zero of=3D/dev/zvol/data/scratch bs=3D1m dd: /dev/zvol/data/scratch: end of device 65+0 records in 64+0 records out 67108864 bytes transferred in 0.037005 secs (1813511443 bytes/sec)
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