Date: Sat, 20 Apr 2019 12:29:17 -0600 From: Ian Lepore <ian@freebsd.org> To: Igor Mozolevsky <igor@hybrid-lab.co.uk> Cc: Hackers freeBSD <freebsd-hackers@freebsd.org> Subject: Re: bhyve VM stopped to boot after moving virtio disks Message-ID: <de0f2df8b187e0f556ac6c259c338a4e0832071f.camel@freebsd.org> In-Reply-To: <CADWvR2iR2THrrx7yK%2BguWKn_Zydnb%2BSFgD5FHFhVk-XeAzyZHQ@mail.gmail.com> References: <alpine.BSF.2.20.1904192255290.97274@puchar.net> <2006ff8da153bbe5e7f620da9260ee1518bd248f.camel@freebsd.org> <alpine.BSF.2.20.1904201729190.99712@puchar.net> <b0d23ad4b9f442a1a7342306443b855407f0765c.camel@freebsd.org> <CADWvR2iR2THrrx7yK%2BguWKn_Zydnb%2BSFgD5FHFhVk-XeAzyZHQ@mail.gmail.com>
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On Sat, 2019-04-20 at 18:45 +0100, Igor Mozolevsky wrote: > On Sat, 20 Apr 2019 at 18:00, Ian Lepore wrote: > > <snip> > > dd absolutely will fail to copy the last block of the source if it > > isn't exactly the blocksize and you didn't specify conv=sync, and > > it > > will return a zero status when doing so. It appears you've > > convinced > > yourself otherwise, but for anyone else reading this thread, be > > aware: > > conv=sync is required to copy the last part of the source if it's > > smaller than the blocksize. > > > Isn't that contrary to the POSIX Spec? Reading the manual [1], it is > implied (from STDERR part of the main section) that dd will read and > write partial blocks, cf. truncated blocks?.. > > 1. https://pubs.opengroup.org/onlinepubs/9699919799/utilities/dd.html > > Damn, why do people trim away the entire useful context of an email thread when replying? I almost don't want to reply to this because at this point all the useful info is gone. In any case, in retrospect, what I said was wrong. Using conv=sync is important when the output is a device that has fixed block sizes where you cannot write a partial block. When the destination is a file where short writes work correctly, it will copy the entire source correctly. -- Ian
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