martinpitt
commented
5 years ago VDO doesn't auto tune the slab size, but it might be cool if cockpit did. Here are some recommended slab sizes based on the physical storage provided to VDO.
Table 29.1. Recommended VDO Slab Sizes by Physical Volume Size https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/storage_administration_guide/vdo-integration#vdo-ig-volume
mvollmer
The minimum size of a VDO backing device is a couple of Gigs. People run into this when playing around with VDO in toy virtual machines, where they add a small 2Gig disk and then get a cryptic error message when trying to put VDO on that.
We should give a clear message that the disk is too small.
More details from Dennis:
The minimum amount of space VDO needs comes down to the index requirements and the slab size (plus some metadata) The smallest index requires about 2.5GB of disk space, this is the default size. The second is the slab size which is 2GB by default and VDO requires 2 slabs, so using the default configuration VDO will need 2.5GB (index) + 2GBx2 (slabs)= 6.5GB. You can set the slab size to 128MB, in doing so, you would then need: 2.5GB (index) + 128MBx2 (slabs) = ~2.7GB of disk space, this is the smallest. I want to point out that with this small size there is only 256MB of physical space to write to not 2.7GB. The index space is reserved for the index. At this small of a volume the index 10x larger than the usable space, but you could also use the same index on a 256TB volume.