Re: [Libguestfs] [PATCH nbdkit] filters: Add copy-on-write filter.

Here's the patch (on top of the preceeding one) which uses a bitmap instead of SEEK_DATA.

+++ b/filters/cow/cow.c @@ -38,20 +38,22 @@ * takes up no space. * * We confine all pread/pwrite operations to the filesystem block - * size. The blk_read and blk_write functions below always happen on - * whole filesystem block boundaries. A smaller-than-block-size - * pwrite will turn into a read-modify-write of a whole block. We - * also assume that the plugin returns the same immutable data for - * each pread call we make, and optimize on this basis. + * size. The blk_* functions below only work on whole filesystem + * block boundaries. A smaller-than-block-size pwrite will turn into + * a read-modify-write of a whole block. We also assume that the + * plugin returns the same immutable data for each pread call we make, + * and optimize on this basis. * - * When reading a block we first check the temporary file to see if - * that file block is allocated or a hole. If allocated, we return it - * from the temporary file. If a hole, we issue a pread to the - * underlying plugin. + * A block bitmap is maintained in memory recording if each block in + * the temporary file is "allocated" (1) or "hole" (0). + * + * When reading a block we first check the bitmap to see if that file + * block is allocated or a hole. If allocated, we return it from the + * temporary file. If a hole, we issue a pread to the underlying + * plugin.