diff --git a/netwerk/cache/nsDiskCacheBlockFile.cpp b/netwerk/cache/nsDiskCacheBlockFile.cpp
index 575a7a2a247b..df378702d69a 100644
--- a/netwerk/cache/nsDiskCacheBlockFile.cpp
+++ b/netwerk/cache/nsDiskCacheBlockFile.cpp
@@ -387,7 +387,8 @@ nsDiskCacheBlockFile::Write(PRInt32 offset, const void *buf, PRInt32 amount)
             mFileSize = PR_MIN(maxPreallocate, PR_MAX(mFileSize, minPreallocate));
         }
         mFileSize = PR_MIN(mFileSize, maxFileSize);
-        mozilla::fallocate(mFD, mFileSize);
+        //  Appears to cause bug 617123?  Disabled for now.
+        //mozilla::fallocate(mFD, mFileSize);
     }
     if (PR_Seek(mFD, offset, PR_SEEK_SET) != offset)
         return false;
diff --git a/netwerk/cache/nsDiskCacheStreams.cpp b/netwerk/cache/nsDiskCacheStreams.cpp
index 875447be291d..a652e37e63bc 100644
--- a/netwerk/cache/nsDiskCacheStreams.cpp
+++ b/netwerk/cache/nsDiskCacheStreams.cpp
@@ -733,8 +733,11 @@ nsDiskCacheStreamIO::FlushBufferToFile()
         if (NS_FAILED(rv))  return rv;
 
         PRInt64 dataSize = mBinding->mCacheEntry->PredictedDataSize();
+// Appears to cause bug 617123?  Disabled for now.
+#if 0
         if (dataSize != -1)
             mozilla::fallocate(mFD, PR_MIN(dataSize, kPreallocateLimit));
+#endif
     }
     
     // write buffer