Date: Sat, 10 Jul 2010 20:38:12 +0200 From: "C. P. Ghost" <cpghost@cordula.ws> To: Patrick Donnelly <batrick@batbytes.com> Cc: questions@freebsd.org Subject: Re: Writes to Hard Disk Going Beyond Capacity Message-ID: <AANLkTinLsM2EEGMZEthZ95YDPnD7Ey20xbeiOR3sXWMp@mail.gmail.com> In-Reply-To: <AANLkTilc1NdTqA91CTxINnAdtyE2ToViPdytTTnwkIm0@mail.gmail.com> References: <AANLkTin9-MymICgQoSjFlKbLXN2Efm5_3r6X1lufqiCo@mail.gmail.com> <87iq4nsr3a.fsf@kobe.laptop> <AANLkTilAuZ3MkzDP5_f7ETk0wcvWR7OLYB8G4c_JVpj9@mail.gmail.com> <AANLkTilc1NdTqA91CTxINnAdtyE2ToViPdytTTnwkIm0@mail.gmail.com>
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On Sat, Jul 10, 2010 at 6:03 PM, C. P. Ghost <cpghost@cordula.ws> wrote: > On Sat, Jul 10, 2010 at 12:53 PM, Patrick Donnelly <batrick@batbytes.com>= wrote: >> On Sat, Jul 10, 2010 at 4:01 AM, Giorgos Keramidas >> <keramida@ceid.upatras.gr> wrote: >>> On Sat, 10 Jul 2010 03:10:31 -0400, Patrick Donnelly <batrick@batbytes.= com> wrote: >>>> Hi List, >>>> >>>> I have a strange problem in a C program I wrote. I open a hard >>>> disk character file (/dev/ad1) and attempt to write over the >>>> entire disk. I expect the last write that would go beyond the >>>> hard disk length (capacity) to return with an error but instead >>>> the write succeeds. =A0This happens for hundreds of gigabytes >>>> beyond the file (hard drive) length. What could be wrong? (This >>>> program works fine on Linux. The last write that would go >>>> beyond the end of the hard drive returns with -1.) >>>> >>>> Thanks for any help, >>> >>> Can we see the exact source code of the program? =A0What you >>> describe might work if the file has holes inside it. >> >> http://www.batbytes.com/destroy >> >> Specifically, after filling the hard drive it will begin to rapidly >> "write" where the throughput of the writes is about 10 GB/s (obviously >> not going to the hard drive). > > Are you aware of short writes? > > static int write_buf (int fd, const char *buf, size_t s) > { > =A0ssize_t r =3D write(fd, buf, s); > =A0if (r =3D=3D -1) > =A0 =A0fprintf(stderr, "write error: %s\n", strerror(errno)); > =A0return r >=3D 0; > } > > What if write(2) returns less than s, but not -1? Or, to be more precise, is it possible that write(2) returns 0 for some reason, perhaps because the device isn't ready and can't accept more data, so it says that it wrote 0 bytes, but that you are free to try again? On Solaris, the write(2) man page says: If a write() requests that more bytes be written than there is room for=97for example, if the write would exceed the process file size limit (see getrlimit(2) and ulimit(2)), the system file size limit, or the free space on the device=97only as many bytes as there is room for will be written. For example, suppose there is space for 20 bytes more in a file before reaching a limit. A write() of 512-bytes returns 20. The next write() of a non-zero number of bytes gives a failure return (except as noted for pipes and FIFO below). http://docs.sun.com/app/docs/doc/816-5167/write-2?l=3Den&n=3D1&a=3Dview Have you tried your program on [Open]Solaris too? What happens there? Perhaps our write(2) isn't entirely IEEE Std 1003.1 compliant? Because write(2) there says: If a write() requests that more bytes be written than there is room for (for example, [XSI] [Option Start] the process' file size limit or [Option End] the physical end of a medium), only as many bytes as there is room for shall be written. For example, suppose there is space for 20 bytes more in a file before reaching a limit. A write of 512 bytes will return 20. The next write of a non-zero number of bytes would give a failure return (except as noted below). (...) http://www.opengroup.org/onlinepubs/000095399/functions/write.html Hmmm... any C/POSIX standards lawyers/specialists here? -cpghost. --=20 Cordula's Web. http://www.cordula.ws/
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