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On 22/05/2015 22:20, Eric Blake wrote: > But I'm worried about a shallow copy. If I start with "base <- > active", where "active" has an explicit zero cluster that is > overwriting an allocated non-zero cluster in "base", and I'm > creating the shallow clone to "base <- copy", then the default of > 'unmap=true' says that bdrv_aio_write_zeroes() may attempt to unmap > the cluster in "copy". At which point, doesn't that mean that > reading from "copy" will dredge up the non-zero data from "base", > which is NOT a faithful mirroring of "active"? No, bdv_aio_write_zeroes+BDRV_REQ_MAY_UNMAP only unmaps if it results in zeroes. In addition, unlike bdrv_aio_discard, bdrv_aio_write_zeroes will do a real write of zeroes if [sector_num, sector_num+nb_sectors) is not aligned to the disk's unmap granularity. Paolo -----BEGIN PGP SIGNATURE----- Version: GnuPG v2 iQEcBAEBCAAGBQJVYzPhAAoJEL/70l94x66DGMYIAKKGQIts+Ve689Iar0P6TmXz BOO0xKJm477PAP9pvUtnGnpt+g2+OqKp0VBTu4F3tbLIQLQOZT1kAZ9oxJ9PMnEa 69c8n86DWdJIgx1lcnOI2wgjaoG65e2K2MH3vYs2Rj6ZD65/FfsF7+oj/R9zCgZm MIMSiW/fbC5EjBQxOTkDWekuB+/iNRtV9K8YBCtmb8ZlF28X3FPrdZnoqcZGtxPL RoGYLckjEuLMLSsDH3Frb1+lfKpCaSlJ203ihQQfXzr/8UBH4zOqPZOTbrhdA1pZ 3fHaW5KWyZ1vjW6bb64kpjMnRwCRxB+uAAY4pUmUCZxGsHxh19hGUeSuOexFvqc= =GUVR -----END PGP SIGNATURE-----
