On Wed, Sep 8, 2010 at 6:35 PM, Anthony Liguori <anth...@codemonkey.ws> wrote: > On 09/08/2010 01:24 PM, Blue Swirl wrote: >> >> Based on these: >> #define TABLE_NOFFSETS (table_size * cluster_size / sizeof(uint64_t)) >> header.image_size<= TABLE_NOFFSETS * TABLE_NOFFSETS * header.cluster_size, >> the maximum image size equals to table_size^2 * cluster_size^3 / >> sizeof(uint64_t)^2. Is the squaring and cubing of the terms >> beneficial? I mean, the size scales up fast to unusable numbers, >> whereas with a more linear equation (for example, allow different L1 >> and L2 sizes), more values could be actually usable. Again, I'm not >> sure if this matters at all. >> >> I think the minimum size should be table_size = 1, cluster_size = 4 >> bytes, 1^2 * 4^3 / 8^2 = 2 bytes, or is the minimum bigger? What's >> the minimum for cluster_size? >> > > 4k. > > The smallest image size is 1GB. There is no upper limit on image size > because clusters can be arbitrarily large.
That's a bit big, for example CD images are only 640M and there were smaller disks. But I guess you mean the smallest maximum size limited by the cluster_size etc, so the actual images may be even smaller. > > >>> It shouldn't matter since any header that is>=16 TB means something >>> mutated, escaped the lab, and is terrorizing the world as a qed >>> monster image. >>> >> >> In the Wiki version this has changed to header_size in clusters. With >> 2GB clusters, there will be some wasted bits. >> > > 2GB clusters would waste an awful lot of space regardless. I don't think > it's useful to have clusters that large. > >> By the way, perhaps cluster_size of 0 should mean 4GB? Or maybe all >> sizes should be expressed as an exponent to 2, then 16 bits would >> allow cluster sizes up to 2^64? >> > > I don't think cluster sizes much greater than 64k actually make sense. We > don't need an image format that supports > 1PB disks. File system developers could want to try images in exabyte ranges. Isn't the purpose of an image format that you can create a virtual disk that can appear to be bigger than the disk space needed?
