Changes between Version 18 and Version 19 of FAQ


Ignore:
Timestamp:
10/03/2009 09:58:56 AM (10 years ago)
Author:
Gabriele Pohl
Comment:

Added note on increasing block size in the future

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  • FAQ

    v18 v19  
    319319=== {{{Unreadable, uncorrectable, pending}}} sectors or {{{Medium error}}} on disk. What's going on? ===
    320320
    321 SCSI and ATA disk drives store data in blocks (sectors) of 512 bytes. Each 512 bytes of user data is stored on the media plus 40 or more bytes of ECC data included in it. These - so called ''ECC Bytes'' - are used internally by the disk firmware for error checking/detection and correction.
     321SCSI and ATA disk drives store data in blocks (sectors) of 512^[#blocksize (1)]^ bytes. Each 512 bytes of user data is stored on the media plus 40 or more bytes of ECC data included in it. These - so called ''ECC Bytes'' - are used internally by the disk firmware for error checking/detection and correction.
    322322
    323323Sometimes the data in a sector gets corrupted.  This can happen because a speck of dust scratched the disk, or because the disk was powered down while writing data to that sector, or for other reasons. Usually the ECC bytes can be used to correct the corrupted data. However if the ECC bytes are inconsistent or can't be used to correct the bad data, then the 512 bytes of data are lost.  Such a sector is called unreadable or uncorrectable.
     
    328328
    329329Normally when an uncorrectable sector is found, the disk puts this onto a 'pending sector list' to indicate that it should be replaced with a spare good sector.  However this replacement won't take place until either the disk can read the data on the bad sector, or is instructed to write new data to that bad sector.
    330 
     330{{{
     331#!html
     332<a name="blocksize">(1)</a> In the future the block size for disks (especially in the terabyte range) will increase from 512 bytes to 4096 bytes.
     333}}}
    331334----
    332335